A 21-year-old university student notices yellowing of his eyes after several days of restricted food intake during examinations. He is otherwise well and has no abdominal pain, dark urine or pale stools. Total bilirubin is 62 micromol/L (3–20), with a conjugated fraction of 7 micromol/L; alanine aminotransferase, alkaline phosphatase, albumin and full blood count are normal, and lactate dehydrogenase is not elevated. Which one of the following is the most likely diagnosis?