A 3-year-old boy from rural Maharashtra presents with severe hypoglycemia (blood glucose 35 mg/dL), hepatomegaly, and lactic acidosis (pH 7.2, lactate 8 mmol/L) following a 6-hour fast. Clinical suspicion is high for a glycolytic enzyme deficiency. Which investigation is most specific for confirming phosphofructokinase (PFK) deficiency?