A patient with progressive muscle weakness and markedly elevated serum creatine kinase (CK) is found to have pure CK-MM elevation. The MB fraction is absent. Muscle biopsy reveals glycogen accumulation. The specific enzyme deficiency most likely causing CK-MM release in the setting of glycogen storage is:

• A Acid maltase (alpha-1,4-glucosidase) deficiency
• B Phosphofructokinase deficiency
• C Branching enzyme deficiency
• D Myophosphorylase deficiency (McArdle's disease) ✓

Explanation

McArdle's disease (type V GSD) results from myophosphorylase deficiency, impairing glycogenolysis in skeletal muscle. Exercise-induced muscle breakdown releases CK-MM (the predominant muscle isoenzyme) without myoglobinuria only in mild cases; severe exertion causes rhabdomyolysis. The ischaemic forearm exercise test shows failure of venous lactate to rise. Acid maltase deficiency (type II) also causes myopathy but targets lysosomes; branching enzyme deficiency causes liver disease predominantly.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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