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 front-to-back.

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 parotid glands.