Click on the alphabet and there you go!

p in biochemistry: The abbreviation for protein. For example, p53 is a protein (53 kilodaltons in size).

p in population genetics: The frequency of the more common of two different alternative (allelic) versions of a gene. (The frequency of less common allele is q).

p53: A protein (53 kilodaltons in size) produced by a tumor-suppressor gene. Like other tumor-suppressor genes, the p53 gene normally controls cell growth. If p53 is physically lost or functionally inactived, this allows cells to grow without restraint.

PA: A physician’s assistant (P.A.) or, in anatomy, PA stands for posteroanterior: from back-to-front. See: PA X-ray.

PA X-ray: An X-ray picture in which the beams pass from back-to-front (posteroanterior). As opposed to an AP (anteroposterior) film in which the rays pass through the body from front-to-back.

Paediatrics: Pediatrics in Great Britain.

Paget's disease: A condition of unknown cause in which the bone formation is out of synchrony with normal bone remodeling.

Pain: It may seem ludicrous to define a sensation that most everyone has experienced (except, for example, people born with complete insensitivity to pain). One standard reference work defines pain as a "more or less localized sensation of discomfort, distress, or agony, resulting from the stimulation of specialized nerve endings."

Pain, ankle: The ankle is a "hinged" joint. The severity of ankle sprains ranges from mild (which can resolve within 24 hours) to severe (which can require surgical repair). Tendinitis of the ankle can be caused by trauma or inflammatory arthritis.

Pain, back: Symptoms in the low back can relate to the bony lumbar spine, discs between the vertebrae, ligaments around the spine and discs, spinal cord and nerves, muscles of the low back, internal organs of the pelvis and abdomen, and the skin covering the lumbar area.

Pain, chest: Chest pain has many cause. One celebrated cause is angina which results from inadequate oxygen supply to the heart muscle due to coronary artery disease or spasm of the coronary arteries. Treatment of angina includes rest, medication, angioplasty, and/or coronary artery bypass surgery.

Pain, elbow: Tendinitis can affect the inner or outer elbow. Treatment includes ice, rest, and medication for inflammation. Bacteria can infect the skin of a scraped (abraded) elbow and cause pain.

Pain, knee: Causes of knee pain include injury, degeneration, arthritis, infrequently infection and rarely bone tumors.

Pains, growing: Mysterious pains in growing children, usually in the legs. These pains are similar to what the weekend gardener suffers from on Monday—an overuse type of problem. If in playing, children exceed their regular threshold, they will be sore, just like an adult. Growing pains are typically somewhat diffuse (vs. focal) and are not associated with physical changes of the area (such as swelling, redness, etc.). The pains are usually relieved by Messages, Tylenol (acetaminophen), or rest. If the pains persist past a week or there are physical changes, the child should be seen by a physician.

Palate: The roof of the mouth. The front portion is bony (hard palate), and the back portion is muscular (soft palate).

Palate, cleft: An opening in the roof of the mouth, due to a failure of the palatal shelves to come fully together from either side of the mouth and fuse during embryonic development.

Palate, hard: The bony part of the roof of the mouth. The hard palate is just in front of the soft palate.

Palate, soft: The muscular part of the roof of the mouth. The soft palate is directly behind the hard palate. It lacks bone and so is soft.

Palindrome in genetics: A palindrome is a word that reads the same in both directions as, for example, the names Eve or Anna. In genetics, a palindrome is a DNA or RNA sequence that reads the same in both directions. The sites of many restriction enzymes that cut (restrict) DNA are palindromes. Palindromic rheumatism is a form of joint inflammation whereby the joints involved appears to change periodically from one region of the body to another and back again.

Palliate: To treat a disease partially and insofar as possible but not cure it completely.

Pallister-Killian syndrome: A condition with multiple malformations at birth and mental retardation due to isochromosome 12p mosaicism (an abnormal chromosome #12 in some cells).

Palmar surface: The palm or grasping side of the hand.

Palpate: To touch or feel. The liver's edge may be palpated.

Palpebra: Medical term for eyelid. The plural is palpebrae.

Palpebral fissure: The opening for the eyes between the eyelids.

Palpitations: Unpleasant sensations of irregular and/or forceful beating of the heart. In some patients with palpitations, no heart disease or abnormal heart rhythms can be found. In others, palpitations result from abnormal heart rhythms (arrhythmias). Arrhythmias refer to heartbeats that are too slow, too rapid, irregular, or too early.

Palsy: Paralysis, generally partial, whereby a local body area is incapable of voluntary movement (motor function). For example, Bell's palsy is localized paralysis of the muscles on one side of the face.

Pancreas: An organ of the digestive system located behind the stomach.

Pancreatic: Having to do with the pancreas.

Pancreatitis: Inflammation of the pancreas. Of many diverse causes, the most common are alcohol and gallstones.

Pancreatic cancer: Cancer of the pancreas. When pancreatic cancer spreads, it usually travels through the lymphatic system. Surgeons often remove lymph nodes near the pancreas to learn whether they contain cancer cells.

Pancreatic insufficiency: Not enough of the digestive enzymes normally secreted by the pancreas into the intestine. Pancreatic insufficiency is a hallmark of cystic fibrosis.

Pancreatic juices: Fluids made by the pancreas that contain digestive enzymes.

Pancytopenia: A shortage of all types of blood cells.

Pancytopenia, Fanconi: A genetic (inherited) disease with progressive decline in blood cells and a tendency to leukemia. Also known as Fanconi anemia.

Pandiculation: One of the more wondrous medical words, pandiculation is the act of stretching and yawning. (If in a public place, you might consider demonstating the versatility of your vocabulary by saying, "Sorry, but I feel the need to pandiculate.")

Panic disorder: Symptoms of panic attack usually begin abruptly and include rapid heartbeat, chest sensations, shortness of breath, dizziness, tingling, and anxiousness. Treatments include several medications and psychotherapy.

PAP test: Microscopic examination of cells collected from the cervix. It is used to detect changes that may be cancer or may lead to cancer, and it can show noncancerous conditions, such as infection or inflammation. Also called Pap smear.

Papillary muscle: Small muscles within the heart which anchor the heart valves.

Papillary tumor: A tumor shaped like a small mushroom with its stem attached to the inner lining of the bladder.

Papilledema: Swelling around the optic nerve, usually due to pressure on the nerve by a tumor.

Papilloma: A benign tumor that projects above the surface of the tissue from which it arises. Papillomas have clearcut borders and are usually small and fairly round.

Papilloma virus, human (HPV): A family of over 60 viruses responsible forcausing warts. The majority of the viruses produce warts on the hands, fingers, and even the face. Most of these viruses are innocuous, causing nothing more than cosmetic concerns. Several types of HPV are confined primarily to the moist skin of the genitals, producing genital warts and elevating the risk for cancer of the cervix. These viruses that cause wartlike growths on the genitals and contrribute to cancer of the cervix are sexually transmitted.

Papillomatosis: A disorder with numerous papillomas.

Papillomatosis, laryngeal: Warty growths on the vocal cords. Most common in young children. Recurrences are, unfortunately, frequent. Remission may occur after several years. Papillomatosis of the larynx can be due to the baby contracting human papilloma virus (HPV) during birth through the vaginal canal from a mother with genital warts (which are due to HPV). Each year, about 300 infants are born with the virus on their vocal cords because of maternal transmission.

Papule: A small solid rounded elevation from the skin.

Paracentric inversion: A basic type of chromosome rearrangement. A segment that does not include the centromere (and so is paracentric) has been snipped out of a chromosome, turned through 180 degrees (inverted), and inserted right back into its original location in chromosome.

Paralysis: Loss of voluntary movement (motor function); may be partial (palsy) or total, such as in botulism.

Paraneoplastic syndrome: A group of symptoms caused by substances produced by certain cancer cells.

Paraphilia: Paraphilias are a variety of complex psychiatric disorders which are manifest as deviant sexual behavior. For example, in men the most common forms are pedophilia (sexual behavior toward children) and exhibitionism (exposing one’s body in public setting). Men with paraphilia are usually treated with psychotherapy, antidepression medications, and medications that alter hormones, particularly testosterone (male sex hormone).

Paraquat lung: Paraquat, a weed killer, selectively accumulates in the lungs and is highly toxic. Once X-ray changes from paraquat are evident in the lungs, death is virtually certain.

Parasite: A plant or animal organism that lives in or on and takes its nourishment from another. Parasite diseases include infections due to protozoa, helminths, or arthropods. For examples: Malaria is caused by plasmodium, a protozoa (a single-cell organism that can only divide within a host organism). Schistosomiasis, another set of very important parasitic diseases, is caused by a helminth (a worm). The arthropods include insects and arachnids (spiders, etc.), a number of which can act as vectors (carriers) of parasitic diseases.

Parasympathetic nervous system: A part of nervous system that serves to slow the heart rate, increase the intestinal and gland activity, and relax the sphincter muscles. The parasympathetic nervous system, together with the sympathetic nervous system (that accelerates the heart rate, constricts blood vessels, and raises blood pressure), constitutes the autonomic nervous system.

Parathormone: Hormone made by the parathyroid gland (behind the thyroid gland in the neck). Parathormone (pronounced para-thor-mone) is critical to calcium and phosphorus balance. Deficiency of parathormone results in abnormally low calcium in the blood (hypocalcemia). Alternative name is parathyrin.

Parathyrin: See Parathormone.

Parathyroid: Gland that regulates calcium, located behind the thyroid gland in the neck. The parathyroid secretes a hormone called parathormone (or parathyrin) that is critical to calcium and phosphorus metabolism. Although the number of parathyroid glands can vary, most people have four, one above the other on each side. They are plastered against the back of the thyroid and therefore at risk for being accidentally removed during thyroidectomy.

Parathyroid hormone: See Parathormone.

Parathyroids, hypoplasia of the thymusand: Also known as the DiGeorge syndrome (DGS), this disorder is characterized by (1) low blood calcium levels (hypocalcemia) due to underdevelopment (hypoplasia) of the parathyroid glands which control calcium; (2) underdevelopment (hypoplasia) of the thymus, an organ behind the breastbone in which lymphocytes mature and multiply; and (3) defects of the heart involving the outflow tracts from the heart. Most cases of DGS are due to a microdeletion in chromosome band 22q11.2. A small number of cases have defects in other chromosomes, notably 10p13. Named after the American pediatric endocrinologist Angelo DiGeorge. Another name for DGS is the third and fourth pharyngeal pouch syndrome.

Parenteral: Not enteric (by the intestine). Something given by injection is parenteral.

Parenteral nutrition: Intravenous feeding. Same as parenteral alimentation.

Paresis: Incomplete paralysis.

Paresis, general: A part of late ("tertiary") syphilis a decade or more after the initial infection, due to chronic inflammation of the covering and substance of the brain (meningoencephalitis) which results in progressive dementia and generalized paralysis.

Paresthesia: An abnormal sensation of the body, such as numbness, tingling, or burning.

Parietal bone: The side bone of the skull.

Parietal lobe: Part of the brain and specifically a section of thecerebral hemisphere.

Parkinson's disease: Parkinson's disease is an abnormal condition of the nervous system caused by degeneration of an area of the brain called the basal ganglia. The disease results in rigidity of the muscles, slow body movement and tremor. Parkinson's disease is also called "paralysis agitans" and "shaking palsy."

Paroxysmal atrial tachycardia (PAT): Bouts of rapid, regular heart beating originating in the atrium (upper chamber of the heart). Due to abnormalities in the AV node "relay station" that lead to rapid firing of electrical impulses from the atrium which bypass the AV node under certain conditions. These conditions include alcohol excess, stress, caffeine, overactive thyroid or excessive thyroid hormone intake, and certain drugs. PAT is an example of an arrhythmia where the abnormality is in the electrical system of the heart, while the heart muscle and valves may be normal.

Parotid gland: The largest of the three major salivary glands, it is located in front and below the ear and behind the jaw bone. The other two glands are the submandibular (submaxillary) and sublingual.

Parotids: Salivary glands situated in front of the ears.

Parotitis: Inflammation of the parotid glands. A classic feature of mumps.

Parrot fever (psittacosis): An infectious disease due to a bacteria (Chlamydia psittaci) contracted from psittacine birds, especially caged birds like parrots but also in turkey processing plants. The name psittacosis comes from the Greek "psittakos" meaning parrot.

Parry’s disease: Toxic multinodular goiter. Named for the English physician Caleb Hillier Parry (1755-1822). also called Plummer’s disease.

Parthenogenesis: Development of a germ cell without fertilization. This is what happens in the formation of some benign ovarian tumors called dermoids (ovarian teratomas). The term "parthenogenesis" comes from the Greek "parthenos" (virgin) + "genesis" (generation). The Greek goddess Artemis (called Parthenos, the virgin) was associated with nymphs who became pregnant and were transformed into beasts.

Partial hysterectomy: In a partial hysterectomy, the uterus is surgically removed but the cervix is left in place. Also called a subtotal hysterectomy.

Parturition: Childbirth.

PAT: Paroxysmal atrial tachycardia.

Patau syndrome: This is trisomy 13syndrome. There are three rather than the normal two chromosomes #13. Children with this syndrome have multiple malformations and mental retardation due to the extra chromosome #13. The malformations commonly include scalp defects, hemangiomas (blood vessel malformations) of the face and nape of the neck, cleft lip and palate, malformations of the heart and abdominal organs, and flexed fingers with extra digits. The mental retardation is profound. The IQ is untestably low. The majority of trisomy 13 babies die soon after birth or in infancy. Named after the late geneticist Klaus Patau (from the University of Wisconsin) who described the extra chromosome in 1960.

Patella: The kneecap.

Patent (noun): A device giving exclusive control and possession. Before the commercialization of biomedical inventions, the word "patent" in this sense had no place in a medical dictionary. Now the patent is the foundation of the biotechnology industry.

Patent (adjective): Pronounced pa’tent with the accent on the first syllable, patent means open, unobstructed, affording free passage. Thus, the bowel can be patent (as opposed to obstructed).

Patent ductus arteriosus (PDA): Before birth, the blood headed from the heart via the pulmonary artery toward the lungs is shunted into the greatest of arteries (the aorta). The shunt is a short vessel called the ductus arteriosus. When the shunt is open, it is said to be patent (pronounced pa’tent). The patent ductus arteriosus (PDA) usually closes at or shortly after birth permitting blood from that moment on to course freely to the lungs. If the ductus remains open (patent), flow reverses and blood from the aorta is shunted into the pulmonary artery and recirculated through the lungs. The PDA may close later spontaneously (on its own) or need to be ligated (tied off) surgically.

Pathologist: A doctor who identifies diseases by studying cells and tissues under a microscope.

Pavlov conditioning: See: Pavlovian conditioning.

Pavlov conditioning: The Russian physiologist Ivan Petrovich Pavlov (1848-1936) conditioned dogs to respond in what proved to be a predictable manner, for example, by first ringing a bell before feeding them and then simply ringing the bell upon which stimulus they would begin to salivate as if they were about to eat.

Pavlov pouch: At different points along the dogs’ digestive tracts, the Russian physiologist Ivan Petrovich Pavlov (1848-1936) surgically created pockets ("Pavlov pouches") from which he could obtain secretions, the aim being to study the physiology of the digestive tract. He did so from the salivary glands down to the stomach, liver and pancreas with considerable success and in 1904 (the 4th year it was awarded) he received the Nobel Prize for "his work on the physiology of digestion, through which knowledge on vital aspects of the subject has been transformed and enlarged."

Pavlov stomach: A Pavlov pouch fashionned surgically from part of the stomach (which is isolated from the rest of the stomach). The pouch opens through a fistula (canal) on to the abdominal wall and permits sampling of the gastric contents. See Pavlov pouch.

PC: Although PC is usually taken to mean personal computer, in the biomedical arena PC also stands for protein C, phosphocreatinine, et al.

PCO: Polycystic ovarian disease (or the Stein-Leventhal syndrome).

PCP: Pneumocystis carinii pneumonia. A parasitic infection of the lungs that is particularly common and life-threatening in immunosuppressed persons. Prophylaxis is available to prevent PCP.

PCR: Stands for Polymerase Chain Reaction, a key technique in molecular genetics that permits the analysis of any short sequence of RNA or DNA without having to clone it.

PDA: Patent ductus arteriosus.

PDR: Physicians’ Desk Reference (please see entry to Physicians’ Desk Reference). PDR less frequently stands for "postdelivery room".

Pectus carinatum: Pigeon-breasted.

Pectus excavatum: Caved-in or funnel chest.

Pediatric: Pertaining to children.

Pediatric arthritis: Arthritis is not just a problem for the retired. It can and does affect children in the form of pediatric arthritis. Also called juvenile arthritis.

Pediatrics: "Pediatrics is concerned with the health of infants, children and adolescents, their growth and development, and their opportunity to achieve full potential as adults." (R.E.. Behrman in Nelson's Textbook of Pediatrics)

Pediculosis: Infested with lice.

Pedigree: In medicine, a pedigree is a family health history diagrammed with a set of international symbols to indicate the individuals in the family, their relationships to one another, those with a disease, etc.

Pedodontics: Chilldren's dentistry.

Pedophilia: Adult sexual activity with children. Considered a form of child abuse. Pedophilia literally means love of children.

Pedigree: In medicine, a pedigree is a family health history diagrammed with a set of international symbols to indicate the individuals in the family, their relationships to one another, those with a disease, etc.

Pellagra: From the Italian pelle, skin and agra. rough. Dermatitis is a feature of pellagra, a syndrome due to deficiency of niacin, one of the B-complex vitamins.

Pelvic: Having to do with the pelvis, the lower part of the abdomen, located between the hip bones.

Pelvic inflammatory disease (PID): Despite its seeming lack of gender, this term is applied to women only. PID refers exclusively to ascending infection of the female genital tract above the cervix.

Pelvis: The lower part of the abdomen located between the hip bones. Organs in the female pelvis include the uterus, vagina, ovaries, fallopian tubes, bladder, and rectum. Pendred syndrome: Hereditary association of congenital deafness and goiter (enlargement of the thyroid gland in the front of the neck) due to a defect in the making of thyroid hormone.

Penetrance: The likelihood a given gene will result in disease. For example, if half (50%) of the people with the neurofibromatosis (NF) gene have the disease NF, the penetrance of the NF gene is 0.5.

Penicillin: Historically, the most famous of antibiotics. Named for the Penicillium fungal mold from which it came. Chicken soup, long known as Jewish penicillin, may in reality have some therapeutic merit.

Penis: Defined by the Oxford English Dictionary as the "intromittent" organ of the male. ("Intromittent " is defined as "That intromits or introduces; having the function of intromission"). In Latin, "penis" originally meant "a tail."

Penis, erection of the: When the penis fills with blood and is rigid. The penis contains two chambers, called the corpora cavernosa, which run the length of the organ, are filled with spongy tissue, and surrounded by a membrane, called the tunica albuginea. The spongy tissue contains smooth muscles, fibrous tissues, spaces, veins, and arteries. The urethra, which is the channel for urine and ejaculate, runs along the underside of the corpora cavernosa. Erection begins with sensory and mental stimulation. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the open spaces. The blood creates pressure in the corpora cavernosa, making the penis expand. The tunica albuginea helps to trap the blood in the corpora cavernosa, thereby sustaining erection. Erection is reversed when muscles in the penis contract, stopping the inflow of blood and opening outflow channels.

Peptic ulcer: A hole in the lining of the stomach, duodenum, or esophagus. A peptic ulcer of the stomach is called a gastric ulcer, an ulcer of the duodenum is a duodenal ulcer, and a peptic ulcer of the esophagus is an esophageal ulcer. A peptic ulcer occurs when the lining of these organs is corroded by the acidic digestive juices which are secreted by the stomach cells.

Peptic ulcer disease is common, affecting millions of Americans yearly. The medical cost of treating peptic ulcer and its complications runs in the billions of dollars annually in the U.S.. Recent medical advances have increased our understanding of ulcer formation. Improved and expanded treatment options are now available.

Percentile: The percentage of individuals in a group who have achieved a certain quantity (such as height, weight, and head circumference) or developmental milestone (such as "walking well" the 50th percentile for which is12 months of age).

Pericardial effusion: Too much fluid within the fibrous sac (the pericardium) that surrounds the heart.

Pericarditis: Inflammation of the lining around the heart (the pericardium) causing chest pain and accumulation of fluid around the heart (pericardial effusion).

Pericardium: A sac of fibrous tissue which surrounds the heart. The inner surface is lined by mesothelial cells and the sac normally contains a small amount of fluid which acts as a lubricant to allow normal heart movement within the chest.

Pericentric inversion: A basic type of chromosome rearrangement. A segment that includes the centromere (and so is pericentric) has been snipped out of a chromosome, turned through 180 degrees (inverted), and inserted back into its original location in chromosome.

Perineum: The area between the anus and the scrotum in the male and between the anus and the vulva (the labial opening to the vagina) in the female. An episiotomy is a surgical procedure to widen the outlet of the birth canal to facilitate delivery of the baby and avoid a jagged rip of the perineum.

Periodontitis: Gum disease, gingivitis.

Peripheral neuropathy: A problem with the functioning of the nerves outside the spinal cord. Symptoms may include numbness, weakness, burning pain (especially at night), and loss of reflexes.

Peristalsis: The rippling motion of muscles in the digestive tract. In the stomach, this motion mixes food with gastric juices, turning it into a thin liquid.

Peritoneal: Having to do with the peritoneum.

Peritoneal dialysis: Technique that uses the patient’s own body tissues inside of the belly (abdominal cavity) to act as a filter. The intestines lie in the abdominal cavity, the space between the abdominal wall and the spine. A plastic tube called a "dialysis catheter" is placed through the abdominal wall into the abdominal cavity. A special fluid is then flushed into the abdominal cavity and washes around the intestines. The intestinal walls act as a filter between this fluid and the blood stream. By using different types of solutions, waste products and excess water can be removed from the body through this process.

Peritoneum: The membrane that lines the abdominal cavity and covers most of the abdominal organs. (From the Greek peri- meaning around + tonos meaning a stretching = a stretching around).

Peritonitis: Inflammation of the peritoneum (The peritoneum is the tissue layer of cells lining the inner wall of the abdomen and pelvis). Peritonitis can result from infection (such as bacteria or parasites), injury and bleeding, or diseases (such as systemic lupus erythematosus).

Perfusion: A chemotherapy technique that may be used when melanoma occurs on an arm or leg. The flow of blood to and from the limb is stopped for a while with a tourniquet, and anticancer drugs are put directly into the blood of the limb. This allows the patient to receive a high dose of drugs in the area where the melanoma occurred.

Perineal surgery: An operation to remove the prostate gland through an incision made between the scrotum and the anus.

Peripheral blood stem cell transplantation: A procedure that is similar to bone marrow transplantation. Doctors remove healthy immature cells (stem cells) from a patient's blood and store them before the patient receives high-dose chemotherapy and possibly radiation therapy to destroy the leukemia cells. The stem cells are then returned to the patient, where they can produce new blood cells to replace cells destroyed by the treatment.

Pernicious anemia: A blood disorder caused by a lack of vitamin B12. Patients who have this disorder do not produce the substance in the stomach that allows the body to absorb vitamin B12.

Pertussis: This is whooping cough's medical name, It is the "P" in DPT vaccine. Immunity from DPT wears off, so many teen-agers and adults get pertussis, first as coughing spasms and then a stubborn dry cough lasting up to 6-8 weeks. Due to a bacteria (Bordetella pertussis). Therapy is supportive and many young infants need hospitalization if the coughing becomes severe. Immunization with DPT (diphtheria-pertussis-tetanus) vaccine provides protection. With pertussis, an ounce of prevention is worth a pound of cure (or, if you are metrically inclined, a gram of prevention is worth a kilo of cure). Have your child immunized!

Pes: Latin for foot.

Pes cavus: Literally a hollow foot, pes cavus is a foot with too high an arch.

Pes planum: Flat feet.

Petechiae: Tiny red spots in the skin which do not blanch when pressed upon. They result from red blood leaking from capillaries. They are not infrequently seen in the legs of patients taking aspirin because its mild blood-thinning effect.

Petit mal: A form of epilepsy with very brief, unannounced lapses in consciousness. Petit mal (little illness in French) seizures are also known as absence seizures.

Phage: Short for bacteriophage, a virus that naturally lives within a bacterial cell. Much used in molecular genetics.

Phagocyte: A cell that can engulf particles such as bacteria and other microorganisms, foreign matter, etc. The principal phagocytes include the neutrophils and monocytes, both types of white blood cells.

Phalanges: The name given to the bones of the fingers by Aristotle in the 4th century B.C. (and since extended to the bones of the toes) because they were arrayed like Greek soldiers for battle. The singular of phalanges is phalanx.

Pharmacogenetics: The convergence of pharmacology and genetics dealing with genetically determined responses to drugs. For example, after the administration of a muscle relaxant commonly used in surgery, a patient may remains apneic incapable of breathing on their own for hours due to a genetically determined defect in metabolizing (processing) the muscle relaxant.

Pharmacology: The study of drugs, their sources, their nature, and their properties.

Pharmacopeia: An official authoritative listing of drugs. Aspirin has, for example, long been in the pharmacopeia.

Pharyngeal: Having to do with the throat (pharynx).

Pharyngitis: Inflammation of the pharynx (hollow tube in the back of the throat about 5 inches long that starts behind the nose and ends at the top of the trachea). Pharyngitis is a common cause for a sore throat.

Pharynx: The hollow tube about 5 inches long that starts behind the nose and ends at the top of the trachea (windpipe) and esophagus (the tube that goes to the stomach).

Phase, resting: More appropriately called interphase. The interval in the cell cycle between two cell divisions when the individual chromosomes cannot be distinguished, interphase was once thought to be the resting phase but it is far from a time of rest for the cell. It is the time when DNA is replicated in the cell nucleus.

Ph.D.: Doctor of Philosophy. (From the New Latin, philosophiae doctor). PhDs are involved in clinical care (as in clinical psychology), biomedical research (as in the Genome Project), health administration and other areas in medicine.

Phalanx: A general term for any one of the bones in the fingers or toes. There are generally three phalanges (distal, middle, proximal) for each digit, except the thumbs and large

Phenocopy: A defect due to an environmental agent that imitates that produced by a specific gene.

Phenotype: The appearance resulting from the interaction of the genetic makeup of a person with the environment. If a child has the gene for osteogenesis imperfecta (brittle bone disease), minimal trauma can cause fractures. Brittle bones are a principal part of the phenotype of osteogenesis imperfecta.

Phenylketonuria (PKU): Inherited inability to process the amino acid phenylalanine. Newborns are screened for PKU. Treatment is a diet low in phenylalanine. Failure of treatment results in mental retardation.

Philadelphia chromosome (Ph): The hallmark of chronic myeloid leukemia (CML). The Ph chromosome is an abbreviated chromosome #22 that was shortchanged in an exchange with chromosome #9. This exchange (a translocation) occurs in a bone marrow cell and causes CML.

Philtrum: The area from below the nose to the upper lip. Normally the philtrum is grooved. In fetal alcohol syndrome, the philtrum is flat.

Phimosis: The foreskin is too tight.

Phlebo-: Means vein.

Phlebitis: Inflammation of a vein. With phlebitis, there is infiltration of the walls of the vein and, usually, the formation of a clot (thrombus) in the vein (thrombophlebitis). Phlebitis in a leg, for example, will cause the leg to swell with edema fluid and feel stiff and painful.

Phlebotomy: Obtaining blood from a vein. This may be for diagnostic tests or treatment (for example, to relieve the iron overload in hemochromatosis).

Phlebotomist: A person who draws blood.

Phocomelia: A congenital malformation (birth defect) in which the hands and feet are attached to abbreviated arms and legs. The word phocomelia combines phoco- (seal) and melia (limb) to designate a limb like a seal's flipper, one consequence of exposure of the developing fetus to thalidomide.

Phobia: Fear.

Phosphate: A form of phosphoric acid. Calcium phosphate makes bones and teeth hard.

Phosphorus: An essential element in the diet and a major component of bone.

Photodynamic therapy: Treatment that destroys cancer cells with lasers and drugs that become active when exposed to light.

Photophobia: Painful oversensitivity to light.

Photosensitivity: The skin is oversensitive to light.

Phototherapy: Treatment with light. For example, a newborn with jaundice may be "put under the lights."

Physical map: A map of the locations of identifiable landmarks on chromosomes. Physical distance is measured in base pairs. The physical map differs from the genetic map which is based purely on genetic linkage data. In the human genome, the lowest-resolution physical map is the banding patterns of the 24 different chromosomes. The highest-resolution physical map will be the complete nucleotide sequence of all chromosomes.

Physician: Only in English is a physician engaged in medicine. In French, for example, a physicien is a physicist.

Physician assistant: A healthcare professional (typically an R.N.) that is licensed to provide patient education, evaluation, a healthcare services. A physician assistant works along with the doctor to provide medical care to a group of patients. Also referred to as a PA.

Physicians’ Desk Reference (PRD): This thick volume—the 1998 PDR runs 3,223 pages in length—is a guide to all the prescription drugs available in the United States. Although not exactly redcommended fare for bedtime reading, the PDR is a key reference to the American pharmacopeia. It is available in many bookstores in the U.S.

Physiatrist: A physician specializing in physical medicine and rehabilitation. Physiatrists specialize in restoring optimal function to people with injuries to the muscles, bones, tissues, and nervous system (such as stroke victims).

Phytanic acid storage disease (Refsum’s disease): A genetic disorder of the fatty acid phytanic acid which accumulates and causes a number of progressive problems including polyneuritis (inflammation of numerous nerves), diminishing vision (due to retinitis pigmentosa), and wobbliness (ataxia) caused by damage to the cerebellar portion of the brain (cerebellar ataxia).

Pia mater: One of the meninges, the pia mater (the term means tender covering) is the delicate innermost membrane enveloping the brain and spinal cord. It is known informally as the pia.

Pianist’s cramp: A dystonia that affects the muscles of the hand and sometimes the forearm and only occurs when playing the piano (or another keyboard instrument such as the harpsichord). Similar focal dystonias have also been called writer’s cramp, typist’s cramp, musician’s cramp, and golfer’s cramp.

Pica: A craving for something not normally regarded as nutritive. For example, dirt. Pica is a classic clue to iron deficiency in children. It also occurs in zinc deficiency. (Pica is Latin for magpie, a bird that gleans all sorts of things for its nest).

Pick's disease: A form of dementia characterized by a slowly progressive deterioration of social skills and changes in personality leading to impairment of intellect, memory, and language.

Pickwickian syndrome: The combination of obesity, somnolence, hypoventilation (underbreathing), and plethoric (red) face named after the "fat and red-faced boy in a state of somnolency" in Charles Dickens' novel The Pickwick Papers. (The same boy is thought by some to have had Prader-Willi syndrome).

Pigeon breast: Prominence of the breast bone (sternum). Medically, pigeon breast is called pectus carinatum.

PID: Pelvic Inflammatory Disease.

Pigment: A substance that gives color to tissue. Pigments are responsible for the color of skin, eyes, and hair.

Pill, the: Slang term for oral contraceptive pill (OCP).

Pilonidal cyst: A special kind of abscess that occurs in the cleft between the buttocks. Forms frequently in adolescence after long trips that involve sitting.

Pilonidal sinus: A dimple in the crease between the buttocks.

Pimples: Oil (sebaceous) glands infected with bacteria, resulting in an inflamed area with pus formation. Pimples are due to overactivity of the oil glands located at the base of the hair follicles, especially on the face, back, chest, and shoulders.

Pineal gland: A small gland located in the brain.

Pineal region tumors: Type of brain tumors.

Pineoblastoma: A type of brain tumor.

Pineocytoma: A type of brain tumor.

Pinguecula: A yellow spot on the white of the eye, usually toward the inside (nose side) of the eye, associated with aging. It looks fatty (in Latin the word pinguiculus means fattish), and is due to an accumulation of connective tissue.

Pinguicula: Alternate spelling of pinguecula. Irrespective of spelling, the accent is on the second syllable which is pronounced gwek.

Pinna: The ear or, to be more precise, the part of the ear that projects like a little wing from the head. In Latin, pinna means wing.

Pit, ear: Tiny pit in front of the ear. Also preauricular pit. A minor anomaly of no great consequence in itself. More common in blacks than whites and in females than males. Can recur in families. The presence of 2 or more minor anomalies in a child increases the probability that the child has a major malformation.

Pituitary adenoma: A benign tumor of the pituitary, the master gland that controls other glands and influences numerous body functions including growth.

Pituitary dwarfism: Short stature due to underpreformance of the pituitary gland (specifically of the anterior pituitary).

Pituitary gigantism: Excessive growth due to overactivity of the pituitary gland (specifically of the anterior pituitary).

Pituitary gland: The main endocrine gland; it produces hormones that control other glands and many body functions, especially growth.

PKU: Short for phenylketonuria.

Placebo: A "sugar pill" or any dummy medication. In a controlled clinical trial, one group may be given a medication and another group a placebo to learn if a difference are due to the medication or to the power of suggestion. Placebos are widely used in drug trials.

Placenta: The organ joining the mother and fetus that permits the provision of oxygen and nutrients to the fetus and the release of carbon dioxide and waste products from the fetus to the mother. The word "placenta" means a flat cake. It is disk-shaped and at full term measures about 7 inches (18 cm) in diameter and a bit less than 2 inches (4 cm) thick. The upper surface of the placenta is smooth while the under surface is rough. The placenta and the fetal membranes are the afterbirth.

Placenta, accessory: An extra placenta separate from the main placenta. Also called a succenturiate or supernumerary placenta. Placenta accreta: The abnormal adherence of the chorionic villi to the myometrium. The vascular processes of the chorion (a fetal membrane that enters into the formation of the placenta) grow directly in the myometrium (the muscular portion of the uterus). Normally there is tissue intervening between the chorionic villi and the myometrium. Here there is not. The word "accreta" comes from the Latin "accretio" from "ad" meaning "to" or "toward" + "crescere" meaning "to grow". Placenta accreta can progress to placenta percreta.

Placenta percreta: The placenta invades the uterine wall. In placenta percreta, the chorionic villi (the vascular processes of the chorion, a fetal membrane that enters into the formation of the placenta) may invade the full thickness of the myometrium (the muscular portion of the uterus) causing an incomplete rupture of the uterus. Or the chorionic villi can go right on through the myometrium and serosa (the outside covering of the uterus) causing complete rupture of the uterus, a catastrophe.

Placenta praevia: A placenta implanted near the outlet of the uterus so that at the time of delivery the placenta would precede (be previous to) the baby. Causes painless bleeding in the last third of pregancy. One reason for a C-section.

Placenta, succenturiate: An extra placenta separate from the main placenta. In anatomy "succenturiate" means accessory to an organ. In this case, a succenturiate placenta is an accessory placenta.

Placenta, supernumerary: A succenturiate or accessory placenta.

Plaque: (1) Dental plaque is the soft accumulation of food debris and bacteria around teeth. These bacteria feed on left over food in the mouth to excrete toxins that irritate the gums and dissolve the bone. Plaque can be removed by proper brushing and flossing at home. Once plaque is left around the teeth for a long time, it acquires minerals from the saliva and from foods which harden it into tartar. Tartar can become as hard as a rock and then can require a dentist or dental hygienist with special tools to remove it. Dental plaque and tartar cause inflammation of the bone surrounding the teeth referred to as "periodontia."

(2) Plaque of an artery refers to hard formation on the artery walls formed by fat and cholesterol deposit over the years. These leads to hardening of the arteries called "atherosclerosis."

Plantar: Having to do with the sole of the foot.

Plantar fasciitis: Inflammation of the plantar fascia (fasciitis), the "bowstring-like" tissue stretching underneath the sole which attaches at the heel.

Plasma: The liquid part of the blood. Plasma is devoid of cells and, unlike serum, has not clotted.

Plasma cells: Special white blood cells that produce antibodies.

Plasmacytoma: A tumor that is made up of cancerous plasma cells.

Plasmapheresis: A procedure whereby plasma (which contains proteins, such as antibodies) is separated and removed from the blood and replaced with another solution, such as saline or albumin.

Plasmid: A self-replicating (autonomous) circle of DNA distinct from the chromosomal genome of bacteria. A plasmid contains genes normally not essential for cell growth or survival. Some plasmids can integrate into the host genome, be artificially constructed in the laboratory, and serve as vectors (carriers) in cloning.

Plasmodium: The parasite guilty in the case of malaria (paludism). Plasmodium is a type of protozoa, a single-celled organism able to divide only within a host cell.

Plastic surgeon: A surgeon who specializes in reducing scarring or disfigurement that may occur as a result of accidents, birth defects, or treatment for diseases (such as melanoma).

Platelets: Platelets are irregular disc-shaped elements of the blood which assist in blood clotting. Other major blood elements include protein, serum and red and white blood cells. Although platelets are classed as blood cells, they are not. They are fragments of a large cells called megakaryocytes (literally, large cells).

Pleiotropic: Multiple effects from a single gene. For example, the Marfan gene is pleiotropic with widespread effects and can cause long fingers and toes (arachnodactyly), dislocation of the lens of the eye, and dissecting aneurysm of the aorta.

Pleomorphic: Many-formed. A tumor may be pleomorphic. Pleomorphic is synonymous with protean (capable of assuming different shapes like the many-formed Greek god Proteus).

Plethoric: Florid, red-faced.

Pleura: The thin covering that protects and cushions the lungs. The pleura is made up of two layers of tissue that are separated by a small amount of fluid.

Pleural effusion: Outpouring of fluid between the two layers of the pleural membranes that cover the lungs.

Pleural space: Although reference is often made to the "pleural space" (one dictionary defines "pleural effusion" as "the presence of fluid in the pleural space"), there is normally only a small amount of fluid between the two layers of the pleura.

Pleurisy: Pain as a result of inflammation of the pleural membrane that envelops the lungs. Pleurisy is typically noted as pain in the involved area of the chest with breathing.

Pleuritis: Inflammation of the pleura (The thin covering that protects and cushions the lungs). The pleura is made up of two layers of tissue that are separated by a small amount of fluid. When the pleura becomes inflamed, it can produce more than the normal amount of fluid in this space. This is called a pleural effusion.

Plumbism: Another name for lead poisoning.

Plummer’s disease: Toxic multinodular goiter. Also called Parry’s disease.

PMR (PolyMyalgia Rheumatica): A disorder of the muscles and joints of older persons characterized by pain and stiffness, affecting both sides of the body, and involving the shoulders, arms, neck, and buttock areas.

PMS (PreMenstrual Syndrome): A combination of emotional, physical, psychological, and mood disturbances that occur after ovulation and normally end with the onset of the menstrual flow.

Pneumatic larynx: A device that uses air to produce sound to help a laryngectomee talk.

Pneumo-: Combining form pertaining to breathing, respiration, the lungs, pneumonia, or air. Pneumo- is derived from the Greek pneuma meaning wind, air, or breath. In French, pneu is a tire (so called because it contains air).

Pneumococcal pneumonia immunization: This vaccine, which prevents one of the most common and severe forms of pneumonia, is usually given only once in a lifetime, usually after the age of 55, to someone with ongoing lung problems (such as chronic obstructive pulmonary disease (COPD) or asthma) or other chronic diseases (including those involving the heart and kidneys). This vaccination would rarely be given to children.

Pneumococcus (Streptococcus pneumoniae): The #1 cause of bacterial pneumonia and otitis media (middle ear infection) and the #3 cause of bacterial meningitis.

Pneumoconiosis: Deposition of particulate matter (such as asbestos and silicon) in the lungs.

Pneumocystis carinii pneumonia (PCP): A parasitic infection of the lungs that is particularly common and life-threatening in immunosuppressed persons. Prophylaxis (preventative treatment) is available to prevent PCP.

Pneumomediastinum: Free air in the mediastinum (space betweens the lungs) which may give rise to pneumothorax or pneumopericardium and compromise the lungs or heart.

Pneumonectomy: An operation to remove an entire lung.

Pneumonia: An infection that occurs when fluid and cells collect in the lungs.

Pneumonia, aspiration: Inflammation of the lungs due to aspiration (the sucking in of food particles or fluids into the lungs).

Pneumopericardium: Air between the heart and the pericardium, the membrane wrapped around the heart.

Pneumothorax: Free air in the chest outside the lung. Pneumothorax can occur spontaneously ("out of the blue"), follow a fractured rib, occur in the wake of chest surgery, or be deliberately induced in order to collapse the lung.

Podiatrist: A nonmedical specialist in caring for the foot.

Poikiloderma atrophicans and cataract: The Rothmund-Thomson syndrome (RTS), a genetic disorder with numerous features affecting skin (premature aging, excess pigmentation, dilated blood vessels),eyes ( uvenile cataract), nose (saddle nose), teeth (maldeveloped), skeletal system (congenital bone defects) hair (abnormal), gonads (underdevelopment) limbs (soft tissue contractures), growth (short stature). blood (anemia) and a tendency to develop a type of bone cancer (osteogenic sarcoma). The RTS gene is on chromosome 8. The syndrome is recessive so to be affected with RTS a child has to have two RTS genes, one from each parent.

Point mutation: A single nucleotide base change in the DNA, as for example in sickle cell disease.

Poison ivy: Poison ivy is a form of "contact dermatitis" or inflammation of the skin resulting from chemicals produced from the poison ivy vine contacting the skin. The chemicals cause an immune reaction producing redness, itching and blistering of the skin.

Poison oak: Poison oak is a form of "contact dermatitis" or inflammation of the skin resulting from chemicals produced from the poison oak plant contacting the skin. The chemicals cause an immune reaction producing redness, itching and blistering of the skin.

Poisoning: Taking a substance which is injurious to health or can cause death. Poisoning is still a major hazard to children despite child-resistant (and sometimes adult-resistant) packaging and dose-limits per container. Please see MedicineNet's Poison Control Centers.

Polio: Short for poliomyelitis.

Polio immunization: The vaccines available for vaccination against polio are OPV (Oral Polio Vaccine) and IPV (Inactivated Polio Vaccine). OPV is still the preferred vaccine for most children. As its name suggests, it is given by mouth. IPV, or Inactivated Polio Vaccine is given as a shot in the arm or leg. Infants and children should be given four doses of OPV. The doses are given at 2 months, 4 months, 6-18 months and 4-6 years of age. Persons allergic to eggs or the drugs neomycin or streptomycin should receive OPV, not the injectable IPV. Conversely, IPV should be given If the vaccine recipient is on long-term steroid (cortisone) therapy, has cancer, or is on chemotherapy or if a household member has AIDS or there is an unimmunized adult in the house.

Poliomyelitis: An acute and sometimes devastating viral disease. There is inflammation of the central nervous system, especially the anterior horn cells of the spinal cord and the brainstem (the portion of the brain between the cerebral hemispheres and spinal cord). Also called infantile paralysis.

Pollen: Small, light, dry protein particles from plants (trees, grasses, and weeds) spread by the wind. Pollen particles are usually the male sex cells of the plant and are smaller than the tip of a pin or less than 40 microns in diameter. Even though pollen is a potent stimulator of allergy. It lodges in the nasal lining tissues (mucus membranes) and other parts of the respiratory tract where it does harm to the person of allergy.

Pollex: The thumb.

Poly: (1) A prefix meaning much or many; (2) An informal term for polymorhonuclear leukocyte (a type of white blood cell).

Polyarteritis nodosa: An autoimmune disease (immune system attacking its own body) characterized by spontaneous inflammation of the arteries (arteritis) of the body. Because arteries are involved, the disease can affect any organ of the body, most commonly muscles, joints, intestines, nerves, kidneys, and skin.

Polyarticular: Involving many joints. As opposed to monoarticular (affecting just one joint). From the Latin "articulus," meaning a joint.

Polycystic kidney disease: Genetic (inherited) disorders characterized by the development of innumerable cysts in the kidneys filled with fluid that replace much of the mass of the kidneys and reduce kidney function leading to kidney failure.

Polycystic ovarian disease: An hormonal problem that causes women to have a variety of symptoms including irregular or no periods, acne, obesity and excessive hair growth. Women with PCO are at a higher risk for uterine cancer (endometrial cancer), diabetes, high blood pressure, and heart disease. With proper treatment, risks can be minimized. PCO is also known as Stein-Leventhal syndrome.

Polycythemia: The opposite of anemia. Too many red blood cells. Polycythemia formally exists when the hemoglobin, red blood cell (RBC) count, and total RBC volume are all above normal.

Polycythemia vera (PV): Overproduction (proliferation) of red blood cells due to bone marrow disease (myeloproferative disorder). PV tends to evolve into acute leukemia or a condition with the marrow replaced by scar tissue (myelofibrosis)

Polydactyly: More than the normal number of fingers or toes.

Polydipsia: Excessive thirst all the time. Polydipsia occurs, for example, in untreated or poorly controlled diabetes mellitus.

Polygenes: Many genes. Eye color is polygenically controlled.

Polygenic diseases: Genetic disorders that are caused by the combined action of more than one gene. Examples of polygenic conditions include hypertension, coronary heart disease, diabetes, and peptic ulcers. Because such disorders depend on the simultaneous presence of several genes, they are not inherited as simply as single-gene diseases.

Polyhydramnios: Too much amniotic fluid.

Polymerase chain reaction (PCR): A key technique in molecular genetics that permits the analysis of any short sequence of RNA or DNA without having to clone it.

Polymerase, DNA: Enzyme that catalyzes (speeds) the polymerization of DNA. DNA polymerase uses preexisting nucleic acid templates and assembles the DNA from deoxyribonucleotides.

Polymerase, RNA: Enzyme that catalyzes (speeds) the polymerization of RNA. RNA polymerase uses preexisting nucleic acid templates and assembles the RNA from ribonucleotides.

Polymorphism: The existence of two (or more) forms of a gene with each form being too common to be due merely to new mutation. Examples of polymorphic genes include sickle cell, thalassemia and G6PD, all of which are believed to have become common because they offer an advantage against malaria.

Polymorphonuclear leukocyte: A type of white blood cell with a nucleus that is so deeply lobated or divided that the cell looks to have multiple nuclei. Informally called a poly.

Polymyalgia rheumatica (PMR): A disorder of the muscles and joints of older persons characterized by pain and stiffness, affecting both sides of the body, and involving the shoulders, arms, neck, and buttock areas.

Polymyositis: An inflammatory disease of muscle that begins when white blood cells, the immune cells of inflammation, spontaneously invade muscles, especially those closest to the trunk or torso, resulting in muscle pain, tenderness and weakness.

Polyp: A mass of tissue that develops on the inside wall of a hollow organ, such as the colon. Polypeptide: A compound consisting of 2 or more amino acids. Amino acids make up polypeptides which make up proteins.

Polyploid: Three or more full sets of chromosomes. A polyploid brain tumor cell might for example have 69 or 92 chromosomes.

Polypsis of the colon: Multiple polyps with a high malignant potential in large bowel. This hereditary condition is also known as polypsis coli and Familial Adenomatous Polyposis (FAP).

Popliteal: Refers to the back of the knee.

Popliteal fossa: The hollow behind the knee.

Popliteal pterygium syndrome: An inherited condition with a web behind the knee. (A pterygium is a winglike triangular membrane.)

Pork tapeworm: Known formally as Taenia solium, contracted from undercooked or measly pork (pork infected with the larval forms of the tapeworm). Can grow to be 3-6 feet (0.9-1.8 m) long in the human intestine. Also known as the armed tapeworm and the measly tapeworm.

Porphyria: A varied series of hereditary diseases with increased formation and excretion of chemicals called porphyrins. One type of porphyria -- acute intermittent porphyria -- may have affected members of the House of Hanover in England including Mad King George who may not have been mad but suffering attacks of porphyria.

Portal vein: A large vein formed by the union of the splenic and superior mesenteric veins. It conveys venous blood to the liver for detoxification before being returned to the circulation via the hepatic veins.

Port-wine stain: A mark on the skin that resembles port wine (porto) in its rich ruby red color. Due to an abnormal aggregation of capillaries, a port-wine stain is a type of hemangioma. it occurs on the face as a sign of Sturge-Weber syndrome.

Positional cloning: Cloning a gene based simply on knowing its position in the genome without any idea of the function of that gene. Because this is the reverse of how things have been traditionally done, it has also been called reverse genetics.

Postmature infant: A baby born 1 week (7 days) or more after the usual 9 months (280 days) of gestation.

Postmenopausal: After the menopause. With increasing longevity, women will soon be postmenopausal for one third of their lives.

Postremission therapy: Chemotherapy to kill leukemia cells that survive after remission induction therapy.

Post-term infant: A baby born 2 weeks (14 days) or more after the usual 9 months (280 days) of gestation, as calculated from the Last Menstrual Period (LMP). This is an important calculation, since, if delivery is delayed 3 weeks beyond term, the infant mortality rate skyrockets to 3 times normal.

Post-traumatic stress: A psychological disorder that develops in some individuals who have had major traumatic experiences (and, for example, have been in a serious accident or through a war). The person is typically numb at first but later has symptoms including depression, excessive irritability, guilt (for having survived while others died), recurrent nightmares, flashbacks to the traumatic scene, and overreactions to sudden noises. Post-traumatic stress became known in the 70s due to the adjustment problems of some Vietnam veterans. It was listed as a diagnostic category by the American Psychiatric Association in 1980. Although the name "post-traumatic stress" was new, the condition was not. It was known as "shell shock" in World War I and "battle fatigue" during World War II.

Pouch, Pavlov: At different points along the dogs’ digestive tracts, the Russian physiologist Ivan Petrovich Pavlov (1848-1936) surgically created pockets ("Pavlov pouches") from which he could obtain secretions, the aim being to study the physiology of the digestive tract. He did so from the salivary glands down to the stomach, liver and pancreas with considerable success and in 1904 (the 4th year it was awarded) he received the Nobel Prize for "his work on the physiology of digestion, through which knowledge on vital aspects of the subject has been transformed and enlarged."

Pound: A measure of weight equal to 16 ounces or, metrically, 453.6 grams. The word "pound" goes back to the Latin "pondo" which meant a "weight" (but one of only 12 ounces). The abbreviation for pound—just to confuse non-pound people—is lb. which stands for "libra" (Latin for pound).

Power-of-attorney for health-care decision-making: See: Proxy. health care.

Prader-Willi syndrome: A condition in children with floppiness (hypotonia), obesity, small hands and feet and mental retardation. It is due to loss of part or all of chromosome 15, specifcally the chromosome 15 from the father. The "fat and red-faced boy in a state of somnolency" described by Charles Dickens in his novel The Pickwick Papers is thought by some to have had Prader-Willi syndrome. (The same boy inspired the naming of the Pickwickian syndrome).

Preauricular tag: Common minor anomaly, a rudimentary tag of ear tissue, often containing a core cartilage, usually located just in front of the ear (auricle). Therefore also called preauricular tag.

Precancerous: Not cancerous, but may become cancerous with time. Precocious puberty: The onset "too early" of secondary sexual characteristics such as breast buds in girls, growth of the penis and thinning of the scrotum in boys and the appearance of pubic hair in both sexes. "Too early" is difficult to define because there is so much normal variation. However, precocious puberty is generally the onset of puberty before age 8 years in girls and age 9 years in boys. Preconceptual: Before conception (of a pregnancy).

Preconceptual counselling: The interchange of information prior to pregnancy. Usually for pregnancy planning and care.

Pregnancy, alcohol in: The consumption of alcohol during pregnancy carries the danger of damaging the fetus and causing fetal alcohol syndrome (FAS) or fetal alcohol effects.

Pregnancy danger from urinary tract infection (UTI): A pregnant woman who develops a UTI should be treated promptly to avoid premature delivery of her baby and other risks such as high blood pressure. Some antibiotics are not safe to take during pregnancy. In selecting the best treatment, doctors consider various factors such as the drug’s effectiveness, the stage of pregnancy, the mother’s health, and potential effects on the fetus.

Pregnancy, ectopic: A pregnancy that is not in the usual place and is located outside the inner lining of the uterus. A fertilized egg settles and grows in any location other than the inner lining of the uterus. The vast majority of ectopic pregnancies occur in the fallopian tube (95%), however, they can occur in other locations, such as the ovary, cervix, and abdominal cavity. An ectopic pregnancy occurs in about 1 in 60 pregnancies. A major concern with an ectopic pregnancy is internal bleeding. If there is any doubt, seek medical attention promptly.

Pregnancy, ectopic, symptoms of: Symptoms of an ectopic pregnancy can often be vague and include vaginal bleeding, abdominal or pelvic pain (usually stronger on one side),shoulder pain, weakness, or dizziness. Weakness, dizziness, and a sense of passing out upon standing can represent serious internal bleeding, requiring immediate medical attention.

Pregnancy danger from fifth disease: Caused by a virus known as parvovirus B 19. Symptoms include low-grade fever, fatigue, a "slapped cheeks rash," and a rash over the whole body. The illness is not serious in children. Pregnant women (who have not previously had the illness) should avoid contact with patients who have fifth disease. The virus can infect the fetus prior to birth. And, while no birth defects have been reported as a result of fifth disease, it can cause the death of the unborn fetus. The risk of fetal death is 5-10% if the mother becomes infected.

Pregnancy planning: Pregnancy planning addresses issues of nutrition, vitamins, body weight, exercise, and potentially harmful medications and illnesses as well as immunizations and genetic counseling.

Preleukemia: A condition in which the bone marrow does not function normally. It does not produce enough blood cells. This condition may progress and become acute leukemia. Preleukemia also is called myelodysplastic syndrome or smoldering leukemia.

Premature contraction of the heart: When a single heartbeat occurs earlier than normal. This phenomenon can be within normal limits or represent a medically significant arrhythmia.

Premature ventricular contractions (PVCs): Contractions of the lower chambers of the heart, the ventricles, which occur earlier than usual, because of abnormal electrical activity of the ventricles. The premature contraction is followed by a pause, as the heart electrical system "resets" itself and the contraction following the pause is usually more forceful than normal. These more forceful contractions are frequently perceived as palpitations.

Prematurity: Historically, the definition of prematurity was 2500 grams (about 5 1/2 pounds) or less at birth. The current World Health Organization definition of prematurity is a baby born before 37 weeks of gestation, counting from the first day of the Last Menstrual Period (LMP).

Premenstrual syndrome (PMS): A combination of emotional, physical, psychological, and mood disturbances that occur after ovulation and normally end with the onset of the menstrual flow.

Prepuce: The fold of skin near the tip of the penis in the uncircumscribed male or the small fold of skin which partially or completely hides the clitoris in the female.

Preventive medicine: Medicine designed to avert and avoid disease. Screening for hypertension and treating it before it causes disease is good preventive medicine. Preventive medicine is a proactive approach.

Priapism: Abnormally persistent erection of the penis in the absence of desire. Named after Priapus, the Greek and Roman god of procreation whose nud statues were placed in fields as scarecrows where their attributes became well known.

Primary biliary cirrhosis (PBC): A liver disease caused by an abnormality of the immune system. Small bile ducts within the liver become inflamed and obliterated. Backup of bile causes intense skin itching and yellowing of the skin (jaundice). Lack of bile decreases absorption of calcium and vitamin D, leading to osteoporosis. Cirrhosis (scarring of the liver) develops over time.

Primary care: The "medical home" for a patient, ideally providing continuity and integration of health care. All family physicians and most pediatricians and internists are in primary care. The aims of primary care are to provide the patient with a broad spectrum of care, both preventive and curative, over a period of time and to coordinate all of the care the patient receives.

Primary dentition: The set of 20 first (deciduous) teeth. The primary dentition is as opposed to the secondary (permanent) dentition. At birth, both sets of dentition are evident by X-ray.

Primary sclerosing cholangitis (PSC): A condition found in association with ulcerative colitis in which the large bile ducts outside the liver become inflamed and obstructed leading to frequent infections and jaundice, causing cirrhosis and sometimes creating the need for liver transplantation.

Primary teeth: The first teeth which are shed and replaced by permanent teeth. The first primary tooth comes in (erupts) at about 6 months of age and the 20th and last one erupts at around 2 1/2 years of age. The primary teeth are replaced usually beginning at about age 6. Also called baby teeth, milk teeth, temporary teeth or deciduous teeth. (In Latin, decidere means to fall off or be shed, like leaves from a tree).

Primitive neuroectodermal tumors: A type of brain tumor. Prenatal diagnosis: Diagnosis before birth. Methods for prenatal diagnosis include ultrasound (of the uterus, placenta and developing fetus), chorionic villus sampling (CVS) to obtain tissue for chromosome or biochemical analysis, amniocentesis to obtain amniotic fluid for the analysis of chromosmes, enzymes, DNA, etc. A growing number of birth defects and diseases are now amenable to prenatal diagnosis. Also called antenatal diagnosis.

Prions: A new type of disease-causing agent, neither bacterial nor fungal nor viral, containing no genetic material, a prion is a protein that occurs normally in a harmless form in the brain. (The word prion was coined from PRoteinaceous + Infectious + the suffix ON meaning a subatomic particle, like a proton or neutron). By folding into an aberrant shape, the normal prion turns into a rogue agent. It then coopts other normal prions to become rogue prions that slowly destroy brain cells until the ravaged brain resembles a sponge. Prions have been held responsible for a number of degenerative brain diseases including mad cow disease, Creutzfeld-Jacob disease, fatal familial insomnia, kuru (a disease transmitted by cannibalism), an unusual form of hereditary dementia (Gertsmann-Straeussler-Scheinker disease) and some cases of Alzheimer's disease. Dr. Stanley B. Prusiner received the 1997 Nobel Prize in physiology or Medicine for his discovery of prions..

Private mutation: A rare mutation found usually only in a single family or a small population. It is like a privately printed book.

Pro-: Prefix (from both Greek and Latin) with many meanings including "before, in front of, preceding, on behalf of, in place of, and the same as." Used as a word, pro of course means professional and, in medicine, it is short for prothrombin.

Probability: Medicine is to a significant degree based on probability theory. Probability in this context is the likelihood of something happening. The abbreviation for probability is P. For example, P<0.05 indicates that the probability of something occurring by chance alone is less than 5 in 100, or 5%. As a matter of fact, P<0.05 is usually taken as the level of biologic significance where a result may be considered meaningful.

Proband: The family member through whom a family medically comes to light. The proband might for example be a baby with Down syndrome. The proband may also be called the index case, propositus (if male) or proposita (if female).

Probe: (1) In surgery, a probe is a slender flexible rod with a blunt end used to explore, for example, an opening to see where it goes. (2) In molecular genetics, a probe is a labeled bit of DNA or RNA used to find its complementary sequence or locate a particular clone like homing in on a needle in a haystack.

PROC: Protein C

Process: In anatomy, a process is a projection from a structure. From the Latin pro-, forward + ceder, to go + a going forward. The process of the mandible is the part of the lower jaw that projects forward.

Proctology: From the Greek proktos meaning the anus or hindparts, proctology deals with anorectal disorders.

Proctosigmoidoscopy: An examination of the rectum and the lower part of the colon using a thin, lighted instrument called a sigmoidoscope.

Product, gene: The RNA or protein that results from the expression of a gene. The amount of gene product is a measure of the degree of gene activity.

Progeria: A disorder characterized by premature aging.

Prognathism: An overly prominent jaw.

Progesterone: A female hormone, progesterone is the principal progestational hormone. Progestational hormones prepare the uterus (the womb) to receive and sustain the fertilized egg.

Prognosis: The probable outcome or course of a disease; the patient's chance of recovery.

Prokaryote: Cell lacking a discrete nucleus and other special subcellular compartments. Bacteria and viruses are prokaryotes. Humans are not prokaryotes, but rather eukaryotes.

Promoter: In molecular biology, a site on DNA to which the enzyme RNA polymerase can bind and initiate the transcription of DNA into RNA.

Pronation: Rotation of the arm or leg inward. In the case of the arm, the palm of the hand will face posteriorly.

Prone: Lying face downward.

Pronucleus: A cell nucleus with a haploid set of chromosomes (23 chromosomes in humans) resulting from meiosis (germ-cell division). The male pronucleus is the sperm nucleus after it has entered the ovum at fertilisation but before fusion with the female pronucleus. Similarly, the female pronucleus is the nucleus of the ovum before fusion with the male pronucleus.

Prophylactic cranial irradiation: Radiation therapy to the head to prevent cancer from spreading to the brain.

Prophylaxis: The prevention of disease.

Propositus: The family member through whom a family medically comes to light. Also called the proband or index case. The feminine of propositus is proposita.

Propylthiouracil (PTU): A drug that blocks the production of thyroid hormones by the thyroid gland. PTU is used to treat hyperthyroidism, to reduce the excessive thyroid activity before surgery and to treat and maintain patients not having surgery.

Prostate acid phosphatase: An enzyme produced by the prostate that is elevated in some patients with prostate cancer.

Prostate cancer: An uncontrolled (malignant) growth of cells in the prostate gland which is located at the base of the urinary bladder and is responsible for helping control urination as well as forming part of the semen. Prostate cancer is the second leading cause of death of males in the U.S.

Prostate gland: A gland in the male reproductive system just below the bladder. It surrounds part of the urethra, the canal that empties the bladder.

Prostate specific antigen: A protein that is elevated in some patients with prostate cancer.

Prostatectomy: The surgical removal of the prostate gland.

Prosthesis: An artificial replacement of a part of the body, such as a tooth, a facial bone, the palate, or a joint.

Prostatitis: Inflammation of the prostate gland.

Prosthodontist: A dentist with special training in making replacements for missing teeth or other structures of the oral cavity to restore the patient's appearance, comfort, and/or health.

Protease: An enzyme that can split a protein into peptides (from whence the protein was originally created).

Protease inhibitor: An agent that can keep a protease from working and splitting a protein into peptides. Protease inhibitors are used in HIV/AIDS treatment.

Protein C: A vitamin K-dependent protein in plasma that enters into the cascade of biochemical events leading to the formation of a clot.

Protein C deficiency: Protein C is a protein in plasma that enters into the cascade of biochemical events leading to the formation of a clot. Deficiency of protein C results in thrombotic (clotting) disease and excess platelets with recurrent thrombophlebitis (inflammation of the vein that occurs when a clot forms). The clot can break loose and travel through the blood stream (thromboembolism) to the lungs causing a pulmonary embolism, brain causing a stroke (cerebrovascular accident), heart causing an early heart attack, skin causing what in the newborn is called neonatal purpura fulminans, the adrenal gland causing hemorrhage with abdominal pain, abnormally low blood pressure (hypotension), and salt loss. Protein C deficiency is due to possession of one gene (heterozygosity) in chromosome band 2q13-14. The possession of two such genes (homozygosity) is usually lethal.

Protein-losing enteropathy: Condition in which plasma protein is lost to excess into the intestine. This can be due to diverse causes including gluten enteropathy, extensive ulceration of the intestine, intestinal lymphatic blockage, and infiltration of leukemic cells into the intestinal wall.

Protein malnutrition: Children are particularly prone to develop protein malnutrition. To grow, children have to consume enough nitrogen-containing food (protein) to maintain a positive nitrogen balance whereas adults need only be in nitrogen equilibrium.

Protein-calorie malnutrition: Severe deficiency of protein + inadequate caloric intake = kwashiorkor.

Proteinuria: Excess protein in the urine. Some protein is normal in the urine. But too much means protein is leaking through the kidney, most often through the glomeruli.

Proteus syndrome: A disturbance of cell growth including benign tumors under the skin, overgrowth of the body, often more on one side than the other (hemihypertrophy), and overgrowth of fingers (macrodactyly). The syndrome is named after the Greek god Proteus the polymorphous who could change his appearance. The "elephant man" (John MerricK) of 19th century England who was thought to have had neurofibromatosis probably had Proteus syndrome.

Proto-oncogene: A normal gene involved in cell division or proliferation which, when altered by mutation, becomes an oncogene that can contribute to cancer.

Prothrobin: A coagulation factor needed for the normal clotting of blood. In the cascade of events leading to the final clot, prothrombin precedes thrombin (and so is a precursor to thrombin). Also called thrombinogen. Prothrombin time: A clotting test, a test done to test the integrity of part of the clotting scheme. Familiarly called the "pro time," the test is the time needed for clot formation after a substance called thromboplastin (+ calcium) has been added to plasma.

Pro time: Prothrombin time.

Protozoa: A single-cell organism that can only divide within a host organism. Malaria is caused by a protozoa: Plasmodium. Other protozoan parasites Giardia and Toxoplasma.

Proximal: The nearer of two or more whatevers. For example, the proximal end of the femur is part of the hip joint. The opposite of proximal is distal.

Proxy, health care: A health care proxy is one form of advance medical directive. Advance medical directives preserve the person’s right to accept or reject a course of medical treatment even after that person becomes mentally or physically incapacitated to the point of being unable to communicate those wishes. There are two basic forms of advance directives:

(1) a living will, in which the person outlines specific treatment guidelines that are to be followed by health care providers; (2) a health care proxy (also called a power of attorney for health-care decision-making) in which the person designates a trusted individual to make medical decisions in the event that he or she becomes too incapacitated to make such decisions. Advance directive requirements vary greatly from one jurisdiction to another and should therefore be drawn up in consultation with an attorney who is familiar with the laws of the particular jurisdiction. (This entry is based upon material from the National MS Society).

Pruritic: Medicalese for itchy. A scab may be pruritic.

Pruritis: Itching. Pruritis can result from drug reaction, food allergy, kidney or liver disease, cancers, parasites, aging or dry skin, contact skin reaction, such as poison ivy, and for unknown reasons.

Pruritus: Itching. Poison ivy causes intense pruritus.

Pseudodementia: A severe form of depression resulting from a progressive brain disorder in which cognitive changes mimic those of dementia.

Pseudogout: Inflammation of the joints caused by deposits of calcium pyrophosphate crystals, resulting in arthritis, most commonly of the knees, wrists, shoulders, hips, and ankles, usually affecting only one or a few joints at a time. True gout is due to a different type of crystal formed by the precipitation of uric acid.

Pseudomembranous colitis: Severe inflammation of the inner lining of the colon. Pseudomembranous colitis is characterized by pus and blood in the stool and often caused by antibiotics.

Pseuodoparalysis, spastic: Better known as Creutzfeldt-Jakob disease (CJD). A dementing disease of the brain. It is believed due to an unconventional (not a bacteria or virus), transmissible agent called a prion. Symptoms of CJD include forgetfulness, nervousness, jerky trembling hand movements, unsteady gait, muscle spasms, chronic dementia, balance disorder, and loss of facial expression. CJD is classified as a spongiform encephalopathy. Most cases occur randomly (sporadically), but inherited forms exist. There is neither treatment nor cure for CJD. Other names for CJD include Creutzfeldt-Jakob syndrome and Jakob-Creutzfeldt disease.

Pseudorubella: Synonymous with Roseola infantum, a viral disease of infants sudden onset of high fever which lasts several and young children with days and then suddenly subsides leaving in its wake a fine red rash. The causative agent is herpesvirus type 6 so the disease is known as Sixth Disease. Also as Exanthem subitum (sudden rash), roseola, roseola infantilis.

Pseudoxanthoma elasticum: (abbreviated PXE), a rare disorder of degeneration of the elastic fibers with tiny areas of calcification in the skin, back of the eyes (retinae), and blood vessels. PXE is inherited from the parents, either as an autosomal recessive or as an autosomal dominant trait. PXE typically causes yellow-white small raised areas in the skin folds, often appearing in the second or third decades of life. These skin abnormalities frequently appear on the neck, armpits, and other areas that bend a great deal (referred to as flexure areas). The face is not affected by PXE. The doctor can often see abnormalities in the back of the eye (retinae) called angioid streaks, which are tiny breaks in the elastin-filled tissue there. These eye abnormalities can lead to blindness. Other areas that can be affected in PXE include the heart which can be affected by atherosclerosis and mitral valve prolapse. Small blood vessels are abnormally fragile in patients with PXE because the blood vessel walls contain elastin and are weakened. This can lead to abnormal bleeding in such areas as the bowel and, very rarely, the uterus. Impairment of circulation to the legs can lead to pains in the legs while walking (claudication).

Psittacosis (parrot fever): An infectious disease due to a bacteria (Chlamydia psittaci) contracted from psittacine birds, especially caged birds like parrots but also in turkey processing plants. The name comes from the Greek psittakos meaning parrot.

Psoriasis: A reddish, scaly rash often located over the surfaces of the elbows, knees, scalp, and around or in the ears, navel, genitals or buttocks. Approximately 10-15% of patients with psoriasis develop joint inflammation (inflammatory arthritis).

Psoriatic arthritis: Joint inflammation associated with psoriasis. Psoriatic arthritis is a potentially destructive and deforming form of arthritis that affects approximately 10% of persons with psoriasis.

Psyche: The mind.

Psychiatrist: A physician specializing in the prevention, diagnosis, and treatment of mental illness.

Psychogenic: Caused by the mind or emotions.

Psychologist: A professional concerned with the mind and behavior. Training in psychology leads to the masters and doctor of philosophy (PhD) degrees.

Psychosis: In the general sense, a mental illness that markedly interferes with a person's capacity to meet life's everyday demands. In a specific sense, it refers to a thought disorder in which reality testing is grossly impaired.

Psychosis, schizophrenia: The most chronic and disabling of the major mental illnesses. Schizophrenia may be one disorder, or it may be many disorders, with different causes. Because of the disorder's complexity, few generalizations hold true for all people who are diagnosed as schizophrenic.

Psychosomatic illness: The mind influences the body to create or exacerbate illness.

PTCA (Angioplasty): Procedure with a balloon-tipped catheter to enlarge a narrowing in a coronary artery. PTCA stands for Percutaneous Transluminal Coronary Angioplasty.

Pterygium: A winglike triangular membrane. Although a pterygium can be anywhere (including behind the knee), it commonly refers to an winglet (of the conjunctiva) extending across the white of the eye toward the inner corner of the eye, caused environmentally by prolonged exposure of the eyes to wind and weather or familially by a single gene.

Ptosis: Downward displacement. Ptosis of the eyelids is drooping of the eyelids.

PTU: Propylthiouracil, an antithyroid medication.

Pubarche: Just as menarche means the time when menstruation begins, pubarche indicates when pubic hair begins.

Puberty: Adolescence. The word puberty derives from the Latin pubertas: coming to the age of manhood.

Pubis: The front center portion of the pelvis.

Public health: Medicine concerned with the health of the community as a whole. Community health.

Pubic symphysis: The joint between the pubic bones at the front of the pelvis.

Pulmonary: Having to do with the lungs. (The word comes from the Latin pulmo for lung).

Pulmonary edema: Fluid in the lungs.

Pulmonary embolism: Sudden closure of a pulmonary artery or one of its branches by a bloodborne clot or foreign material that plugs the vessel. Pulmonary embolus: A blood clot within the lung's pulmonary artery. An embolus causes an embolism. In this case, the embolus, a clot or foreign material, has been carried through the blood into the pulmonary artery or one of its branches, plugging the vessel. (Embolus is from the Greek embolos for plug or wedge)

Pulmonary hypertension: High blood pressure in the pulmonary arteries. Normally, the pressure in the pulmonary arteries is low (compared to that in the aorta). Pulmonary hypertension can irrevocably damage the lungs.

Pulmonary insufficiency: Pulmonary here refers to the valve between the right ventricle of the heart and the pulmonary artery. If this valve is insufficient (incompetant) in its performance, blood sloshes back from the pulmonary artery into the right ventricle.

Pulmonary stenosis: The pulmonary valve is too tight so that the flow of blood from the right ventricle of the heart into the pulmonary artery is impeded.

Pulmonary valve: One of the four valves in the heart, the pulmonary valve stands at the opening from the right ventricle in the pulmonary artery trunk. It lets blood head in the right direction (toward the lungs) and keeps it from sloshing back from the pulmonary artery into the heart.

Pulse: The pulse is the rhythmic dilation of an artery resulting from beating of the heart. It is often measured by feeling the arteries of the wrist The word pulse is from the Latin pulsus meaning, among other things, a beating. The ancient Greeks and Romans recognized that the pulse in arteries (as at the wrist) was connected with the beating of the heart.

Pump-oxygenator: A machine that does the work both of the heart (pump blood) and the lungs (oxygenate the blood). Used, for example, in open heart surgery. Blood returning to the heart is diverted through the machine before returning it to the arterial circulation. Also called a heart-lung machine.

Punch biopsy: A punch is an instrument for cutting and removing a disk of tissue. A punch biopsy of the skin may for example be done to make the diagnosis of a malignancy.

Puncture wound: An injury from piercing or penetrating with a pointed object. Any puncture wound through tennis shoes (as with a nail) has a high risk of infection because the foam in tennis shoes can harbor a bacteria (Pseudomonas).

Pupil: The opening of the iris. The pupil may appear to open (dilate) and close (constrict) but it is really the iris that is the prime mover; the pupil is merely the absence of iris.

Purine: One of the two classes of bases in DNA and RNA. The purine bases are guanine (G) and adenine (A). Uric acid, the offending substance in gout, is a purine end-product.

Purpura: A hemorrhage area in the surface of the skin. The appearance of an individual area of purpura varies with the duration of the lesions. Early purpura is red and becomes darker, then purple, and brown-yellow as it fades.

Purpura, anaphylactoid: See Purpura, Henoch-Schonlein.

Purpura, Henoch-Schonlein (HSP): HSP is a form of blood vessel inflammation, a vasculitis that affects small arterial vessels in the skin (capillaries) and the kidneys. HSP results in skin rash associated with joint inflammation (arthritis) and cramping pain in the abdomen. HSP frequently follows a bacterial or viral infection of the throat or breathing passages and is an unusual reaction of the body’s immune system to this infection. HSP occurs most commonly in children. HSP is generally a mild illness that resolves spontaneously, but sometimes it can cause serious problems in the kidneys and bowels. Treatment is directed toward the most significant area of involvement. Joint pain can be relieved by antiinflammatory medications, such as aspirin or ibuprofen. Some patients can require cortisone-related medications, such as prednisone, especially those with significant abdominal pain or kidney disease. Also known as anaphylactoid purpura.

Pus: A thick whitish-yellow fluid which results from the accumulation of white blood cells (WBCs), liqified tissue and cellular debris. Pus is commonly a site of infection or foreign material in the body.

PVC: Abbreviation for premature ventricular contraction.

PXE: Abbreviation for pseudoxanthoma elasticum, a rare disorder of degeneration of the elastic fibers with tiny areas of calcification in the skin, back of the eyes (retinae), and blood vessels. PXE is inherited from the parents, either as an autosomal recessive or as an autosomal dominant trait. PXE typically causes yellow-white small raised areas in the skin folds, often appearing in the second or third decades of life. These skin abnormalities frequently appear on the neck, armpits, and other areas that bend a great deal (referred to as flexure areas). The face is not affected by PXE. The doctor can often see abnormalities in the back of the eye (retinae) called angioid streaks, which are tiny breaks in the elastin-filled tissue there. These eye abnormalities can lead to blindness. Other areas that can be affected in PXE include the heart which can be affected by atherosclerosis and mitral valve prolapse. Small blood vessels are abnormally fragile in patients with PXE because the blood vessel walls contain elastin and are weakened. This can lead to abnormal bleeding in such areas as the bowel and, very rarely, the uterus. Impairment of circulation to the legs can lead to pains in the legs while walking (claudication).

Pycnodysostosis: An inherited disorder of the bone. that causes short stature and abnormally dense brittle bones. Due to a defect in an enzyme: cathepsin K. The French artist Toulouse-Lautrec is thought to have pycnodysostosis. Also spelled pyknodysostosis with a "k".

Pyelo: Short for pyelonephritis.

Pyelogram: X-ray study of the kidney especially showing the pelvis (urine-collecting basin) of the kidney and the ureter.

Pyelonephritis: Bacterial infection of the kidney. Pyelonephritis can be acute (sudden) or chronic (slow, subtle, and stubborn). It is most often due the ascent of bacteria from the bladder up the ureters to infect the kidneys.

Pyloric stenosis: Narrowing (stenosis) of the outlet of the stomach so that food cannot pass easily from it into the duodenum, resulting in feeding problems and projectile vomiting. The obstruction can be corrected by a relatively simple surgical procedure.

Pylorus: The outlet of the stomach.

Pyoderma gangrenosum: An ulcerating condition of skin resulting in heaped borders with a typical appearance. Pyoderma gangrenosum appears to be mediated by the immune system, but the exact cause is unknown. The lesion(s) usually begin as a soft nodule on the skin which proceeds to ulcerate. The ulcer enlarges and the skin at the edge is purple-red. Ulcers can become quite large. This condition is associated with several other diseases, some of which are ulcerative colitis, Crohn’s disease, rheumatoid arthritis, leukemia, and cryoglobulinemia. Pyoderma gangrenosum is usually responsive to corticosteroids.

Pyrimidine: One of the two classes of bases in DNA and RNA. The pyrimidine bases are thymine (T) and cytosine (C) in DNA and thymine (T) and uracil (U) in RNA.

Pyuria: Pus in the urine. Pyuria is a sign of inflammation often related to infection.