A 21-year-old man presents to the clinic with mild jaundice that appears intermittently, particularly during periods of fasting and stress. He reports no abdominal pain, pruritus, or dark urine. On examination, he has icteric sclerae but no hepatosplenomegaly. Laboratory studies reveal: total bilirubin 3.2 mg/dL (normal: 0.1–1.2 mg/dL), direct (conjugated) bilirubin 0.4 mg/dL (normal: 0–0.3 mg/dL), and normal levels of ALT, AST, and alkaline phosphatase. Which of the following biochemical defects most likely explains his presentation?