Neuromuscular Disorders USMLE Step 1 Practice Question

A 17-year-old boy presents with 2-year history of progressive proximal muscle weakness affecting hip and shoulder girdles. He ambulates independently but reports difficulty rising from chairs. Vital signs: BP 118/76, HR 92, RR 16, Temp 37°C. Cardiac evaluation reveals early dilated cardiomyopathy with ejection fraction 45%. Serum creatine kinase is elevated at 2,800 U/L. Muscle biopsy shows reduced but not absent dystrophin expression. Cognitive function remains normal. Which diagnosis best explains these findings?