A 19-day-old girl is reviewed for jaundice that has persisted since discharge from hospital. Her mother reports that the urine has stained the nappy dark and the stools have become pale grey; the infant is otherwise feeding well and gaining weight. Examination shows mild hepatomegaly. The total bilirubin is 126 micromol/L (reference <20), with a conjugated fraction of 82 micromol/L (reference <5). Which one of the following is the most likely diagnosis?