A 5-day-old female neonate born at 35 weeks gestation to an Indian mother presents with progressive jaundice. Birth weight 2.2 kg. Antenatal course unremarkable; mother's blood group A-positive, baby's blood group A-positive. Direct Coombs test negative. Serum bilirubin 16 mg/dL (total), 0.3 mg/dL (direct). Hemoglobin 13.5 g/dL, reticulocyte count 2%, peripheral smear normal. Mother is breastfeeding; baby has passed meconium and urine normally. What is the most likely cause of jaundice?