A 35-year-old man from rural India presents with jaundice, dark urine, and abdominal pain for 2 weeks. Serum bilirubin is elevated at 8 mg/dL with predominantly direct hyperbilirubinemia. AST and ALT are markedly elevated (>1000 IU/L). He has no prior history of hepatitis. Which investigation is most appropriate to confirm acute hepatitis A infection?