A 32-year-old man from Delhi presents with jaundice, dark urine, and pale stools for 10 days. Serum bilirubin is 8.2 mg/dL (conjugated 6.1 mg/dL), ALT 1200 IU/L, AST 980 IU/L, and ALP 320 IU/L. Anti-HAV IgM is negative. Abdominal ultrasound shows normal liver echotexture with no features of cirrhosis. Which investigation is most appropriate to establish the etiology of acute hepatitis in this patient?