A 18-hour-old term Indian newborn presents with visible jaundice. The mother is Rh-negative and did not receive anti-D prophylaxis during pregnancy or at delivery. The structure marked **B** in the diagram represents jaundice appearing within the first 24 hours of life. Which of the following is the MOST LIKELY diagnosis and the MOST URGENT next step in management?

Explanation

## Why Rh isoimmunization with hemolytic disease of newborn; perform direct Coombs test and initiate phototherapy if bilirubin is rising rapidly is right Jaundice appearing within the first 24 hours of life is ALWAYS PATHOLOGIC and never physiologic (which peaks day 3–5). The clinical scenario of an Rh-negative unsensitized mother with an Rh-positive infant is a classic risk for Rh isoimmunization and hemolytic disease of the newborn (HDN). The direct Coombs test detects maternal IgG antibodies coating fetal red blood cells, confirming immune hemolysis. Per AAP 2022 guidelines, phototherapy (blue light 460–490 nm) is initiated based on the nomogram for gestational age and risk category, with exchange transfusion reserved for rapidly rising bilirubin or levels exceeding 25 mg/dL. Rh isoimmunization is the most severe form of HDN and is preventable with anti-D prophylaxis at 28 weeks and within 72 hours post-delivery (Nelson 21e Ch 124). ## Why each distractor is wrong - **Physiologic jaundice; reassure parents and recheck bilirubin at 48 hours of life**: Physiologic jaundice peaks on day 3–5 of life, NOT within the first 24 hours. Jaundice in the first 24 hours is ALWAYS pathologic and requires urgent evaluation, not reassurance alone. - **ABO incompatibility; no intervention needed as it is always self-limited**: While ABO incompatibility is a cause of early jaundice (mother O, baby A or B), it is typically milder than Rh disease and still requires evaluation and possible phototherapy. The clinical context (Rh-negative mother) points to Rh isoimmunization. Moreover, "no intervention" is dangerous—all early jaundice requires workup and management per AAP nomogram. - **Breast milk jaundice; increase feeding frequency and recheck bilirubin in 1 week**: Breast milk jaundice typically appears after day 3–5 and peaks around day 2 weeks. It is NOT a cause of jaundice within the first 24 hours. Waiting 1 week is inappropriate for early pathologic jaundice, which risks kernicterus (bilirubin encephalopathy with basal ganglia deposition, choreoathetoid cerebral palsy, sensorineural hearing loss, and upward gaze palsy). **High-Yield:** Jaundice in the first 24 hours = ALWAYS PATHOLOGIC; think hemolytic disease (Rh or ABO), sepsis, TORCH, or G6PD—never physiologic. Direct Coombs + phototherapy per AAP nomogram is the standard approach. [cite: Nelson Textbook of Pediatrics 21e Ch 124; AAP Clinical Practice Guideline 2022]