A 3-year-old child from rural Maharashtra presents with recurrent episodes of severe hypoglycemia, lactic acidosis, and hepatomegaly. Biochemical workup shows elevated lactate (8 mmol/L), normal blood glucose during fasting, but profound hypoglycemia within 2–3 hours of fasting. The child's parents report similar symptoms in a deceased sibling. Which investigation is most appropriate to confirm a defect in glycolysis?