Neuromuscular Disorders USMLE Step 1 Practice Question

A 4-year-old boy is brought by his parents for delayed motor milestones and progressive difficulty rising from the floor. He uses his hands to push up his thighs when standing (Gowers sign). Vital signs: BP 95/60, HR 88, RR 20, Temp 37°C, SpO2 98%. Physical examination reveals calf pseudohypertrophy, proximal leg weakness, and exaggerated lumbar lordosis. Serum creatine kinase is markedly elevated at 5,800 IU/L. No respiratory distress noted. Which diagnosis best explains these findings?