Metabolism USMLE Step 1 Practice Question
A 19-year-old man presents with severe muscle cramps and early fatigue during sprinting and weightlifting. He reports dark urine after intense exercise. Vital signs at rest are normal (BP 120/80, HR 72, RR 16). Physical examination is unremarkable. Serum creatine kinase is elevated at 850 U/L. During forearm ischemic exercise testing, venous lactate fails to rise appropriately despite normal glucose levels. Urine myoglobin is positive. Which enzyme deficiency best explains these findings?
Answer choices
- AGlucose 6 phosphatase
- BAcid alpha glucosidase
- CPyruvate kinase
- DMuscle glycogen phosphorylaseCorrect answer
- EVery long chain acyl CoA dehydrogenase
- FPhosphofructokinase-1
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