A 3-day-old breastfed full-term infant has visible jaundice. Total serum bilirubin is 11.5 mg/dL, with unconjugated (indirect) bilirubin 10.8 mg/dL. Reticulocyte count is normal and Coombs test is negative. What is the most likely diagnosis?

Answer and explanation

Correct answer: C. Physiological jaundice of the newborn

  1. Step 1: Physiological neonatal jaundice appears after 24 hours of life (typically peaking on days 3-5 at levels < 12-15 mg/dL) due to transient immaturity of hepatic UDP-glucuronosyltransferase activity and increased red blood cell turnover.
  2. Step 2: Normal reticulocyte counts and negative direct Coombs test exclude active immune hemolysis.
Answer
Physiological jaundice of the newborn.
Why not the others: 'Biliary atresia' presents after 2 weeks of life with conjugated (direct) hyperbilirubinemia, acholic stools, and dark urine; 'ABO hemolytic disease' presents within the first 24 hours of life with a positive Coombs test; 'G6PD deficiency' typically causes severe acute hemolytic episodes with elevated reticulocytes.

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