A B C D E F G H I J K L M N O P Q R S T U V W XYZ

xeroderma pigmentosum

A genetic syndrome with extreme sensitivity to sunlight leading to progressive disfigurement and skin cancer. There are eight subgroups, some of which exhibit progressive neurological deterioration, both motor and cognitive. Inheritance is autosomal recessive with an incidence of 1 in 100,000. See also deSanctis-Cacchione syndrome.

X-linkage

Sex-linkage; characteristics, traits, or disorders carried on genes on the X chromosome. Males are more often affected, as they have only one X chromosome. These traits are never transmitted from father to son.

X-linked hydrocephalus syndrome

A genetic syndrome with hydrocephalus (excess fluid in the brain), short flexed (cortical) thumbs, spasticity (usually diplegia [paralysis on both sides of the body]), and mental retardation. Inheritance is X-linked recessive. Ultrasound may be used to attempt prenatal diagnosis in families considered at risk.

XLMR

X-linked mental retardation.

XO syndrome

See Turner syndrome.

XYY syndrome

Additional Y chromosomes contribute to tall stature and mild mental retardation. Severe acne, psycho-sexual problems, and aberrant behavior are also reported. Originally this syndrome was thought to be associated with a genetic predisposition to criminal behavior, but later studies showed that subjects were not more aggressive or criminally inclined but simply more liable to get caught and go to prison (where the genetic studies were being performed) than more intelligent criminals. Incidence is 1 in 840 with a recurrence risk of 1%.

Yale Children's Inventory (YCI)

A 63-item parent questionnaire that assesses attention, behavior, and cognitive problems in children. The YCI is part of the Yale Neuropsychological Assessment Scales (YNPAS), which also includes a Child's Personal Data Inventory (CPDI) and a Teacher's Behavior Rating Scale (TBRS).

YCI

See Yale Children's Inventory.

yeast

See Candida albicans.

Z score

A standard score with a mean of zero and a standard deviation of 1. If a test had a mean of 63 and a standard deviation of 9, a raw score of 72 would he equivalent to a Z score of 1. The Z score is the number of standard deviations a given value is from the test mean.

Zantac

See ranitidine.

zebra

A rare disease or condition. A term used in medicine to describe a diagnosis that is extremely rare. The aphorism "Uncommon presentations of common diseases are more common than common presentations of uncommon diseases" is an anti-zebra statement. It suggests that more time, effort, and energy should be expended pursuing statistically more probable conditions before pursuing more expensive explorations of possible zebras.

Zellweger syndrome

Cerebrohepatorenal syndrome; a syndrome caused by the absence of a peroxisomal enzyme and manifested by atypical brain structures, hypotonia (decreased muscle tone), enlarged liver, large fontanels (soft spots), heart disease, and kidney cysts. Most babies with this syndrome are born breech and live only a few months. The hypotonia and atypical brain structures are associated with severe developmental delay. Inheritance is autosomal recessive. Decreased enzyme and increased fatty acid levels in amniotic fluid cells allow prenatal diagnosis.

zero reject

The element of full inclusion that requires special education services for all students with disabilities to be provided in the general classroom.

Z-plasty

A tendon-lengthening procedure that involves sectioning half of the tendon (e.g., tibialis posterior) lower down and the contralateral (opposite) half of the tendon higher up and then suturing the overlapped ends with the extremity (e.g., foot) in neutral.

zygomatic bone

Zygoma; cheekbone.