A 28-year-old woman from rural Uttar Pradesh presents with acute-onset jaundice, severe abdominal pain, and vomiting at 8 weeks of gestation. Serum ALT is 2400 IU/L, bilirubin 9.5 mg/dL, PT-INR 2.8, and albumin 2.8 g/dL. Anti-HAV IgM and HBsAg are both negative. Which is the most appropriate confirmatory test for the suspected diagnosis?