A 6-week-old exclusively breastfed infant presents with jaundice. He is otherwise well, gaining weight appropriately, and has pale yellow stools and dark urine. Examination is unremarkable. Direct bilirubin is 25 µmol/L (Total bilirubin 180 µmol/L). What is the most likely cause of prolonged jaundice in this infant, requiring further investigation according to NICE CG98?