A 3-day-old male neonate, born to an O Rh-negative mother and Rh-positive father, presents with progressive jaundice. Birth weight 3.2 kg. On examination: hepatosplenomegaly, petechial rash, and poor feeding. Serum bilirubin 18 mg/dL (unconjugated), hemoglobin 10.5 g/dL, reticulocyte count 8%, and direct antiglobulin test (Coombs) is strongly positive. Peripheral blood smear shows nucleated RBCs and polychromasia. What is the most likely diagnosis?

Explanation

## Diagnosis: Hemolytic Disease of the Newborn (HDN) — Rh Incompatibility ### Clinical Presentation The constellation of findings — Rh-negative mother with Rh-positive baby, early-onset jaundice (day 3), hepatosplenomegaly, petechial rash, and anemia — is pathognomonic for **Rh incompatibility**. ### Laboratory Confirmation | Feature | Rh Incompatibility | ABO Incompatibility | |---------|-------------------|---------------------| | **Coombs test** | Strongly positive (IgG antibodies) | Weakly positive or negative | | **Hemoglobin** | Severe anemia (often <10 g/dL) | Mild anemia | | **Reticulocyte count** | Markedly elevated (>8%) | Mildly elevated | | **Nucleated RBCs** | Prominent | Absent or few | | **Onset** | Day 1–3 (severe) | Day 2–7 (milder) | | **Hepatosplenomegaly** | Marked | Minimal | | **Petechiae/purpura** | Common | Rare | **Key Point:** Rh incompatibility causes **IgG sensitization** from prior pregnancy or transfusion; the second Rh-positive baby is affected. ABO incompatibility (naturally occurring IgM antibodies) affects the firstborn. ### Pathophysiology 1. Rh-negative mother previously sensitized (prior pregnancy or transfusion) → IgG anti-D antibodies 2. IgG crosses placenta → binds fetal RBC antigens 3. Extravascular hemolysis in fetal spleen and liver 4. Severe anemia, hepatosplenomegaly, extramedullary hematopoiesis 5. Unconjugated hyperbilirubinemia (immature conjugation) 6. Risk of kernicterus if untreated **High-Yield:** Rh incompatibility is now **rare in developed countries** due to antenatal anti-D prophylaxis (RhoGAM), but remains common in resource-limited settings and in unsensitized mothers without prophylaxis. ### Management 1. Phototherapy (bilirubin >18 mg/dL at 3 days) 2. Exchange transfusion if bilirubin continues to rise or signs of kernicterus emerge 3. Monitor hemoglobin and reticulocyte count 4. Prevent future sensitization: anti-D prophylaxis in next pregnancy **Clinical Pearl:** The strongly positive Coombs test and nucleated RBCs are the diagnostic hallmarks — they distinguish Rh incompatibility from ABO incompatibility and other causes of neonatal jaundice.