A 28-year-old woman from Delhi presents with a 6-month history of progressive muscle weakness, myalgia, and dark-colored urine after intense exercise. She reports that her symptoms worsen with strenuous activity and improve with rest. Serum creatine kinase is 3200 U/L, and urine myoglobin is positive. A muscle biopsy shows glycogen accumulation. During ischemic forearm exercise test, venous lactate fails to rise (remains <1 mmol/L) while ammonia increases markedly. Which enzyme deficiency is most likely responsible for her symptoms?