In physiologic jaundice, which bilirubin fraction is elevated?

• A. Biliverdin
• B. Unconjugated bilirubin
• C. Delta bilirubin
• D. Conjugated bilirubin

Answer: (B)

The main concept is that physiologic jaundice in newborns is driven by an immature ability to conjugate bilirubin. Most of the bilirubin in this situation is unconjugated (indirect) bilirubin, which is bilirubin that has not yet been processed by the liver to become water-soluble. In newborns, the enzyme that conjugates bilirubin (UDP-glucuronosyltransferase) is underdeveloped, and the rapid turnover of fetal red cells increases bilirubin production. Because unconjugated bilirubin is fat-soluble, it travels in the blood bound to albumin and only after uptake by the liver can it be conjugated. When conjugation can’t keep up with production, unconjugated bilirubin accumulates, producing the characteristic pattern of physiologic jaundice. The conjugated form would rise in conditions affecting bile excretion, such as liver disease or obstruction, not in physiologic jaundice. Delta bilirubin, which is bilirubin bound to albumin, appears with longer-standing jaundice, and biliverdin is just an immediate precursor that is usually not elevated as a serum fraction in this context.