A 3-year-old boy from rural Maharashtra presents with recurrent episodes of muscle pain and weakness after playing for 2–3 hours. His mother reports dark-colored urine during these episodes. Physical examination reveals hepatomegaly. Serum lactate is elevated (8 mmol/L; normal <2), and venous blood gas shows metabolic acidosis. Muscle biopsy shows accumulation of glycogen with normal enzyme histochemistry. Which enzyme deficiency is most likely responsible for this clinical presentation?