Master the board step 3

Master the Boards: USMLE Step 3
Basic Science Correlate
Macrophage
Biliverdin
reductase
Heme
Biliverdin
Bilirubin
Heme
oxygenase
Enterohepatic
circulation
(via portal system)
Bile Duct
Stercobilin
Glucuronic acid
removed by
bacteria
Urobilinogen
(Stercobilinogen)
Intestine
Bilirubin in the Newborn
••
Hemoglobin breaks down to bilirubin.
••
Newborns have low levels of glucuronosyltransferase, the enzyme that
connects or “conjugates” unconjugated bilirubin to glucose so that it can be
excreted through feces. Higher levels of unconjugated bilirubin are needed
during development, when it can cross the placenta and be removed from
the fetus by the mother.
••
The RBCs of newborns also have a shorter life span. Breakdown of RBCs
releases unconjugated bilirubin.
Diagnostic Testing
If jaundice presents in the first 24 hours, workup includes:
··
Total and direct bilirubin
··
Blood type of infant and mother: Look for ABO or Rh incompatibility.
··
Direct Coombs test
··
CBC, reticulocyte count, and blood smear: Assess for hemolysis.
··
Urinalysis and urine culture if elevated direct bilirubin: Assess for sepsis.
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