A 23-year-old man has noted a yellow color to his sclerae for the past 2 weeks. On physical examination, he has generalized jaundice. He has the physique of a bodybuilder. Laboratory studies show serum total bilirubin, 5.6 mg/dL; ALT, 117 U/L; AST, 103 U/L; alkaline phosphatase, 148 U/L; albumin, 5.5 g/dL; and total protein, 7.9 g/dL. Which of the following substances is he most likely to be using?