A 32-year-old man with newly diagnosed pulmonary tuberculosis is started on standard four-drug regimen (HRZE). After 2 months of therapy, he develops sudden-onset jaundice, dark urine, and right upper quadrant pain. Liver function tests show ALT 320 U/L, AST 380 U/L, and total bilirubin 4.2 mg/dL. Which investigation is most appropriate to determine the causative antitubercular drug responsible for hepatotoxicity?