A 32-year-old man from rural India presents with jaundice, malaise, and dark urine for 10 days. Liver function tests show ALT 2800 IU/L, AST 2200 IU/L, bilirubin 8.2 mg/dL, and INR 1.2. He denies alcohol use and has no risk factors for blood-borne viruses. Which investigation is most appropriate to establish the diagnosis of acute viral hepatitis and identify the causative agent?