A 23-year-old man has recurrent episodes of muscle cramps during intense exercise. During an episode, plasma lactate rises from 1.2 to 1.5 mmol/L (reference 0.5–2.2 mmol/L), whereas plasma ammonia rises from 35 to 180 μmol/L (reference 15–45 μmol/L). Muscle biopsy shows excessive glycogen with normal branching. Which defect best explains the metabolic response?
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