Webster's New World Medical Dictionary

anemia, refractory
Anemia that is unresponsive
to treatment.
anemia, sickle cell
A genetic blood disorder
caused by the presence of an abnormal, sickle-
shaped form of hemoglobin. These hemoglobin
molecules tend to aggregate after unloading oxygen,
forming long, rod-like structures that force the red
cells to assume a sickle shape. Unlike normal red
cells, which are usually smooth and malleable, the
sickle red cells cannot squeeze through small blood
vessels. When the sickle cells block small blood ves-
sels, the organs are deprived of blood and oxygen.
This leads to periodic episodes of pain and damages
the vital organs. Sickle red cells die after only about
10 to 20 days, instead of the usual 120 days or so.
Because they cannot be replaced fast enough, the
blood is chronically short of red cells, causing ane-
mia. The gene for sickle cell anemia must be inher-
ited from both parents for the illness to occur in
children. A child with only one copy of the gene may
have sickle-cell traits but no symptoms of illness.
See also 
sickle cell trait.
anencephaly
Absence of the cranial vault and of
most or all of the cerebral hemispheres of the brain,
a lethal malformation. Anencephaly is due to imper-
fect development of the neural tube, the structure
that gives rise to the central nervous system, during
very early pregnancy. The upper end of the neural
tube fails to close. The risk of all neural tube
defects, including anencephaly, is decreased if the
mother’s diet during pregnancy contains ample folic
acid. See also 
neural tube defect.
anesthesia
Loss of feeling or awareness, as when
an anesthetic is administered before surgery.
anesthesiologist
A physician or, less often, a
dentist who is specialized in the practice of anesthe-
siology.