A 28-year-old man with a history of recurrent episodes of severe hypoglycemia (blood glucose 28–35 mg/dL) during prolonged fasting is evaluated. Fasting blood glucose is low, but serum glucose does not rise appropriately after 90 minutes of intravenous glucagon infusion (0.5 mg/min). Serum lactate and alanine are normal. Which investigation would best distinguish between hepatic gluconeogenic enzyme deficiency and glycogen synthase deficiency?