A term neonate develops jaundice at 36 hours. Transcutaneous bilirubin is 16 mg/dL. Blood type is A Rh-positive; mother is O Rh-positive. The neonate has a positive DAT (Coombs test). Peripheral smear shows microspherocytes. What is the mechanism of hemolysis?
- A Rh incompatibility with anti-D IgG crossing the placenta
- B ABO incompatibility with maternal anti-A IgG crossing the placenta ✓
- C G6PD deficiency triggered by oxidative stress
- D Hereditary spherocytosis with osmotic fragility
Explanation
ABO incompatibility is the most common cause of significant hemolytic disease in the newborn, occurring when mother is group O and the baby is group A or B. Group O mothers naturally have IgG anti-A and anti-B antibodies (unlike group A or B mothers whose antibodies are predominantly IgM), which cross the placenta and coat fetal red cells. The DAT is weakly positive, and microspherocytes are seen on smear due to partial phagocytosis of IgG-coated red cells. Rh incompatibility requires prior sensitization and an Rh-positive mother would not be affected by anti-D. G6PD deficiency gives a negative DAT.
Reference: Ghai Essential Pediatrics, 10th ed.
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