Neuromuscular Disorders USMLE Step 1 Practice Question
A 17-year-old boy presents with slowly progressive proximal muscle weakness and exertional dyspnea since age 12. Vital signs: BP 118/76, HR 88, RR 18, temp 37°C, SpO2 98%. He remains ambulatory without assistive devices. Serum creatine kinase is elevated at 2,400 U/L. Muscle biopsy demonstrates reduced but not absent dystrophin expression on immunohistochemistry. There is no cardiac arrhythmia on ECG. Which of the following mutations is most likely present?
Answer choices
- ACTG repeat expansion in DMPK
- BDystrophin in frame mutationCorrect answer
- CSMN1 deletion
- DDystrophin frameshift mutation
- EPMP22 duplication
- FDysferlin gene mutation causing limb-girdle muscular dystrophy type 2B
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