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McArdle Disease – Myophosphorylase Deficiency — SCE Neurology MCQ

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HardPeripheral Neuropathy & NeuromuscularMcArdle Disease – Myophosphorylase DeficiencySCE Neurology

A 30-year-old man presents with exercise-induced myalgia and recurrent episodes of dark brown urine (myoglobinuria) since childhood. He has no weakness between episodes. CK is normal between episodes but rises to >50,000 IU/L during attacks. Muscle biopsy shows absent myophosphorylase activity on histochemistry. What is the diagnosis?

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Correct answer: B — McArdle disease (glycogen storage disease type V)

McArdle disease is caused by myophosphorylase deficiency (autosomal recessive, PYGM gene), preventing glycogen breakdown in muscle. Patients develop exercise intolerance, myalgia, cramps, and rhabdomyolysis with myoglobinuria during anaerobic exercise. The 'second wind' phenomenon (symptoms improve after initial exercise as fatty acid oxidation compensates) is characteristic. Forearm exercise test shows no rise in venous lactate with a normal ammonia rise. A: DMD has progressive weakness from childhood. C: Pompe causes progressive proximal myopathy with respiratory failure. D: Mitochondrial myopathy has different exercise test profile. E: BMD has progressive weakness.

Reference: ABN Metabolic Myopathy Guidelines; EFNS Metabolic Myopathy Guidelines