Gilbert's syndrome

A 25-year-old man comes to his family medicine PA seeking evaluation of jaundice. Two days ago, the patient participated in a marathon race. He had a similar episode in college after an episode of binge drinking. Past medical history is notable for seasonal allergies and hepatitis A infection, which he acquired one-year ago while travelling overseas. In the office, his temperature is 37.5°C (99.5°F), blood pressure is 118/65 mmHg, and pulse is 67/min. Physical examination shows scleral icterus and diffuse jaundice. Cardiac, pulmonary, and abdominal exams are noncontributory. Urinalysis reveals no significant findings. The patient’s blood work reveals the following findings:  

Laboratory value	Result
Hemoglobin	14.2 mg/dL (reference range: 13.5-17.5 mg/dL)
Leukocyte count	6,800/mm3 (reference range: 4,500-11,000/mm3)
Total bilirubin	4.1 mg/dL (reference range: 0.1-1 mg/dL)
Conjugated bilirubin	0.2 mg/dL (reference range: 0.0-0.3 mg/dL)
Alanine aminotransferase	14 U/L (reference range: 8-40 U/L)
Aspartate aminotransferase	17 U/L (reference range: 8-40 U/L)

Which of the following best explains the underlying cause of this patient’s condition?