Webster's New World Medical Dictionary

xeroderma pigmentosum
A genetic disease that
is characterized by such extraordinary sensitivity to
sunlight that it results in the development of skin
cancer at a very early age. Abbreviated XP. Children
Xx
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with XP can only play outdoors safely after nightfall.
XP is due to defective repair of damage done to DNA
by ultraviolet (UV) light. Whereas normal persons
can repair UV-induced damage by inserting new
bases into the DNA, XP patients cannot. A person
with XP develops severe sunburn and eye irritation
within minutes of exposure to sunlight. Other fea-
tures of XP include very dry skin (xeroderma), blis-
ters on the skin, heavy freckling, and dark spots on
the skin. XP is inherited in an autosomal recessive
manner. Defects in multiple genes have been identi-
fied that lead to XP. Avoiding UV light and using the
highest level of sunscreen possible when exposure
cannot be avoided helps prevent complications.
xerophagia
Having a tendency to eat a dry diet.
xerophthalmia
Dry eyes. Xerophthalmia can be
associated with systemic diseases, such as Sjogren’s
syndrome, systemic lupus erythematosus, and
rheumatoid arthritis; deficiency of vitamin A; and
use of some medications. It results from inadequate
function of the lacrimal glands, which produce
tears. When xerophthalmia is due to vitamin A defi-
ciency, the condition begins with night blindness
and conjunctival xerosis (dryness of the eye mem-
branes), progresses to corneal xerosis (dryness of
the cornea), and in its late stages develops into ker-
atomalacia (softening of the cornea). Treatment
depends on the severity of the condition and ranges
from artificial tears and ointments to plugging of the
tear ducts. Also known as conjunctivitis arida.
xerosis
Abnormal dryness of the skin, mucous
membranes, or conjunctiva (xerophthalmia). There
are many causes of xerosis, and treatment depends
on the particular cause.
xerostomia
Dry mouth. Xerostomia can be asso-
ciated with systemic diseases, such as Sjogren’s syn-
drome, systemic lupus erythematosus, and
rheumatoid arthritis; and it can be a side effect of
medication and poor dental hygiene. Xerostomia
results from inadequate function of the salivary
glands, such as the parotid glands. Treatment
involves adequate intake of water, use of artificial
saliva, and good dental care. Untreated, severe dry
mouth can lead to increased levels of tooth decay
and thrush.
xiphoid process
The lower part of the breast-
bone. The xiphoid process has no particular func-
tion and ranges in size from miniscule to several
inches in length.
X-linked
A gene on the X chromosome that is
located on the X chromosome. An X-linked disorder
is associated with or caused by a gene on the X
chromosome.