A 6-week-old exclusively breastfed infant presents with jaundice. He was born at term with normal birth weight and is otherwise well, feeding effectively and gaining weight appropriately along the 50th centile. Urine is pale yellow, and stools are pigmented yellow/green. Examination reveals jaundice but no hepatosplenomegaly. Bilirubin level is 180 μmol/L, predominantly unconjugated. What is the most likely cause of the jaundice?