Webster's New World Medical Dictionary

SPF
1
Sun protection factor.
2
Specific-
pathogen free.
sphenoid bone
A prominent, irregular, wedge-
shaped bone at the base of the skull. The sphenoid
bone has been called the “keystone” of the cranial
floor because it is in contact with all of the other
cranial bones.
spherocytosis, hereditary
A genetic disorder
of the red blood cell membrane that is character-
ized by anemia, jaundice, and enlargement of the
spleen (splenomegaly). Abbreviated HS. In HS, the
red cells are smaller, rounder, and more fragile
than normal. They have a spherical shape rather
than the biconcave-disk shape of normal red cells.
These fragile red cells (spherocytes) tend to get
trapped in narrow blood passages, particularly in
the spleen. If this occurs, they break up (hemolyze)
where they have lodged, leading to hemolytic ane-
mia. The clogging of the spleen with red cells almost
invariably causes splenomegaly. The breakup of the
red cells releases hemoglobin, and the heme part
gives rise to bilirubin, the pigment of jaundice.
Often patients with HS also have iron overload due
to the excess destruction of iron-rich red cells. HS
often appears in infancy or early childhood, causing
anemia and jaundice. Four different abnormalities
in red cell membrane proteins have been identified
in patients with HS, including deficiencies in the
blood cell membrane proteins known as spectrin
and ankyrin. Depending upon the specific form, HS
may be inherited as an autosomal dominant or
recessive trait. Diagnosis is made via laboratory
study of the blood. HS is most common in people of
northern European descent. Treatment involves
removing the spleen (splenectomy). Although the
red cell defect persists after splenectomy, the
hemolysis ceases and the prognosis after splenec-
tomy is for a normal life expectancy. Also known as
congenital hemolytic jaundice, severe atypical sphe-
rocytosis, spherocytosis type II, ankyrin deficiency,
erythrocyte ankyrin deficiency, ankyrin-R deficiency,
and ankyrin1 deficiency.
sphincter
A muscle that surrounds and, by its
contraction, closes a normal opening such as that
from the intestinal tract or the urinary tract. Damage
to the anal and urethral sphincters can cause fecal
and urinary incontinence, respectively. Sphincters
tend to be ring-like and, when contracted, to con-
strict the opening. From the Greek for “that which
constricts.”