xanthelasmaA yellowish deposit of
fatty material that is visible in the skin
around the eyes. Xanthelasmas are com-
mon in elderly people and are usually
of no more than cosmetic importance.
However, in younger people they may
be associated with hyperlipidaemias, in
which there is excess fat in the blood.
Xanthelasmas may be removed, if nec-
essary, by a simple surgical procedure
under a local anaesthetic. Any associated
hyperlipidaemia must also be treated.
(See also xanthomatosis.)
xanthomaA yellowish deposit of fatty
material in the skin, often on the elbow
or buttock. They may be associated with
hyperlipidaemias(see xanthomatosis).
xanthomatosisA condition in which
deposits of yellowish, fatty material
develop in various parts of the body,
particularly in the skin, internal organs,
corneas of the eyes, brain, and tendons.
The deposits may occur only in the eye-
lids (see xanthelasma). A key feature of
xanthomatosis is the tendency for fatty
material to be deposited in the linings
of blood vessels, leading to generalized
atherosclerosis. Xanthomatosis is often
associated with hyperlipidaemias.
Treatment aims to lower the levels of
fats in the blood by means of a diet that
is low in cholesterol and high in poly-
unsaturated fat, and by drug treatment.
X chromosomeA sex chromosome, of
which every normal female body cell
has a pair. Male body cells have 1 X and
1 Y chromosome; each sperm carries
either an X or a Y chromosome. Abnor-
mal genes located on X chromosomes
cause X-linked disorders.
xeroderma pigmentosumA rare, in-
herited skin disease. The skin is normal
at birth, but photosensitivity (extreme
sensitivity to sunlight) causes it to
become dry, wrinkled, freckled, and pre-
maturely aged by about the age of 5.Noncancerous skin tumours and skin
cancersalso develop. Xeroderma pig-
mentosum is often accompanied by
related eye problems, such as photo-
phobiaand conjunctivitis.
Treatment of the condition consists of
protecting the skin from sunlight. Skin
cancers are usually treated surgically or
with radiotherapy.
xerophthalmiaAn eyedisorder in which
vitamin Adeficiency causes the conjunc-
tiva and cornea to become abnormally
dry. Without treatment, xerophthalmia
may progress to keratomalacia, a condi-
tion in which severe damage is caused
to the cornea.
xerostomiaAbnormal dryness of the
mouth, which can cause bad breath and
may predispose the sufferer towards
tooth decay (see caries, dental). Xerosto-
mia is sometimes a symptom of Sjögren’s
syndrome. (See also mouth, dry.)
xipamideA thiazide diureticdrug used
to treat oedema(accumulation of fluid
in tissues) and high blood pressure.
Side effects may include dizziness and
mild gastrointestinal disturbances.
xiphisternumAn alternative name for
the xiphoid process, the small, leaf-
shaped projection that forms the lowest
of the 3 parts of the sternum.
X-linked disordersSex-linked genetic
disordersin which the abnormal gene or
genes (the causative factors) are loca-
ted on the X chromosome. Almost all
affected people are males. Haemophilia,
fragile X syndrome. and colour vision
deficiencyare examples.
X-raysA form of electromagnetic radia-
tion of short wavelength and high energy.
X-rays are widely used in medicine for
diagnosis and treatment because they
can be used to image bones, organs,
and internal tissues.
X-rays are produced artificially by
bombarding a heavy metal tungsten tar-
get with electrons, in a device known as
an X-ray tube. Low doses of the X-rays
that are emitted are passed through
body tissue and form images on film or
a fluorescent screen. The X-ray image,
also known as a radiograph or roent-
genogram, shows the internal structure
of the area that is being examined.
Dense structures, such as bone, absorbXANTHELASMA X-RAYSX