A 26-year-old woman presents with 1 week of fever, fatigue, poor appetite, nausea, vomiting, and dark urine. Over the past 2 days, fever subsided but jaundice rapidly worsened with somnolence. Lab results: ALT 660 IU/L, AST 450 IU/L, total bilirubin 250 μmol/L. Which of the following tests would be most valuable for further diagnosis?