X: Although the
letter X is used as a symbol in various ways (such as X-rays,
the X-axis of a graph, etc.), today "the X " usually refers to
the X chromosome.
X chromosome: The sex chromosome found twice in normal females
and singly, along with a Y chromosome, in normal males.
X-linked: On the X chromosome. "Linked" in genetics does not
mean merely associated. An X-linked gene travels with the X
chromosome and therefore is part of the X chromosome.
X-ray: High-energy radiation. It is used in low doses to
diagnose diseases and in high doses to treat cancer.
X-ray, AP: An X-ray picture in which the beams pass from
front-to-back (anteroposterior). An AP film is as opposed to a
PA (posteroanterior) film in which the rays pass through the
body from back-to-front.
X-ray, lateral: An X-ray picture taken from the side.
X-ray, PA: An X-ray picture in which the beams pass from
back-to-front (posteroanterior). By contrast an AP (anteroposterior)
film is one in which the rays pass through the body from
Xanthelasma: Tiny 1-2 mm yellowish plaques that are slightly
raised on the skin surface of the upper or lower eyelids.
Xanthelasma is caused by tiny deposits of fat in the skin and is
often associated with abnormal blood fat levels (hyperlipidemia).
Xanthoma: Yellowish firm nodules in the skin frequently
indicating underlying disease, such as diabetes, disorder of
fats (lipid disorder or hyperlipidemia), or other conditions.
Xeroderma: Abnormally dry skin.
Xerophagia: Eating a dry diet.
Xeropthalmia: Dry eyes. Also, called conjunctivitis arida.
Xeropthalmia can be associated with systemic diseases (such as
Sjogren’s syndrome, systemic lupus erythematosus, rheumatoid
arthritis, scleroderma, sarcoidosis, amyloidosis,
hypothyroidism, and others) or deficiency of vitamin A.
Xeropthalmia results from inadequate function of the lacrimal
glands which supply water to produce tears.
Xerostomia: Dry mouth. Xerostomia can be associated with
systemic diseases (such as Sjogren’s syndrome, systemic lupus
erythematosus, rheumatoid arthritis, scleroderma, sarcoidosis,
amyloidosis, hypothyroidism, and others). Xerostomia results
from inadequate function of the salivary glands, such as the