A 3-year-old boy from rural Maharashtra presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly during a 6-hour fast. His parents report recurrent episodes of irritability, sweating, and tremor when meals are delayed. On examination, he has a doll-like face with full cheeks and a protuberant abdomen. Serum lactate is elevated at 8 mmol/L (normal <2), and uric acid is 9.2 mg/dL. A liver biopsy shows excessive glycogen accumulation. Which glycolytic enzyme deficiency is most likely responsible for this clinical picture?