A 3-year-old child presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly. The child is asymptomatic during fasting but develops symptoms 2–3 hours after meals. Serum lactate is elevated at 8 mmol/L (normal <2), and uric acid is 9.2 mg/dL. Which investigation is most appropriate to confirm the suspected diagnosis of glycogen storage disease type I (Von Gierke disease)?