A 2-year-old boy from Mumbai presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly during a 4-hour fast. On examination, he has a "doll-like" face with full cheeks, short stature, and prominent 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 with normal glycogen structure. Which enzyme deficiency is most likely responsible for this clinical presentation?