Neurodegenerative Diseases USMLE Step 1 Practice Question
A 17-year-old boy presents with slowly progressive proximal muscle weakness and bilateral calf hypertrophy over 5 years. Vital signs: BP 118/76, HR 88, RR 16, Temp 37°C, SpO2 98%. He remains ambulatory without assistive devices. Serum creatine kinase is 3,200 U/L. His cousin with an identical clinical presentation lost ambulation by age 6. Notably, cardiac examination reveals no murmurs, and echocardiography shows normal ventricular function. Which genetic mutation best explains this milder disease course in this patient?
Answer choices
- ALoss of upper and lower motor neurons
- BAutoimmune destruction of motor end plates
- CComplete absence of dystrophin from a frameshift mutation
- DPartially functional dystrophin from a nonframeshift mutationCorrect answer
- ETriplet repeat expansion affecting a chloride channel
- FLimb-girdle muscular dystrophy from sarcoglycan defect
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.