A 3-year-old child from rural Maharashtra presents with recurrent episodes of severe hypoglycemia, lactic acidosis, and hepatomegaly. Biochemical analysis shows elevated lactate (8 mmol/L, normal <2), normal glucose initially but rapid depletion on fasting, and elevated alanine. A defect in pyruvate metabolism is suspected. Which investigation is most appropriate to confirm a defect in the pyruvate dehydrogenase complex?