Webster's New World Medical Dictionary

yolk bone
See 
zygoma.
yolk sac
The membrane outside the human
embryo. The yolk sac is connected by a tube, the
yolk stalk or omphalomesenteric duct, through the
umbilical opening to the embryo’s midgut. The yolk
sac serves as an early site for the formation of
blood, and in time it is incorporated into the prim-
itive gut of the embryo.
yolk stalk
See 
omphalomesenteric duct.
youth
The time between childhood and maturity.
yeast vaginitis
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Z chromosome
A sex chromosome in certain
animals, such as chickens, turkeys, and moths. In
humans, males are XY and females are XX, but in
animals with a Z chromosome, males are ZZ and
females are WZ.
zebra
In medicine, a very unlikely diagnostic
possibility. It comes from an old saying used in
teaching medical students about how to think logi-
cally in regard to the differential diagnosis: “When
you hear hoof beats, think of horses, not zebras.”
For example, when someone develops a mild tran-
sient cough, a virus infection is the most logical and
likely cause, and tuberculosis is a zebra.
Zellweger syndrome
One of a group of related
diseases called peroxisome biogenesis disorders
(PBD), inherited conditions that damage myelin in
the brain and also affect the metabolism of particu-
lar substances. Features include enlargement of the
liver, high levels of iron and copper in the blood, and
vision disturbances. It can be caused by mutations in
a number of different genes. There is no cure for
Zellweger syndrome. The prognosis is poor, with
death usually occurring within 6 months.