A 28-year-old woman from Delhi presents with acute onset of severe muscle pain, dark urine, and myoglobinuria following intense exercise at a gym. Serum creatine kinase is 8,500 U/L (normal <200). Blood glucose is 78 mg/dL, lactate is 1.2 mmol/L (normal), and venous pH is 7.38. Muscle biopsy shows subsarcolemmal glycogen accumulation and absent acid maltase activity. Which glycolytic/glycogenolytic enzyme deficiency would explain the normal lactate despite severe muscle breakdown?