Gilbert's syndrome

A 25-year-old man comes to his outpatient provider 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
Leukocyte count	6,800/mm3
Total bilirubin	4.1 mg/dL
Conjugated bilirubin	0.2 mg/dL
Alanine aminotransferase	14 U/L
Aspartate aminotransferase	17 U/L

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