Neurodegenerative Diseases USMLE Step 1 Practice Question
A 15-year-old boy presents with progressive gait instability and recurrent falls over 2 years. Vital signs: BP 118/76, HR 92, RR 16, temp 37°C. Examination reveals pes cavus deformity, absent ankle reflexes, extensor plantar responses, decreased vibration sense distally, and wide-based gait. Proprioception remains intact. Echocardiography shows hypertrophic cardiomyopathy. Serum vitamin B12 and folate levels are normal. Which genetic mutation best explains this constellation of findings?
Answer choices
- ALoss of dystrophin in skeletal muscle
- BAutosomal recessive GAA repeat expansion causing mitochondrial dysfunctionCorrect answer
- CAutosomal dominant degeneration of Purkinje cells
- DMutation in ATP7B causing copper accumulation
- EPostinfectious autoimmune demyelination of peripheral nerves
- FX-linked recessive mutation in PCDH1 causing cerebellar ataxia
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