Neuromuscular Disorders USMLE Step 1 Practice Question

A 5-year-old boy presents with progressive difficulty climbing stairs and frequent falls over the past 6 months. He uses his hands to push himself upright from the floor (Gowers sign). Vital signs: HR 98/min, BP 105/68 mmHg, RR 20/min, Temp 37°C. Physical examination reveals calf pseudohypertrophy bilaterally and proximal lower extremity weakness. Serum creatine kinase is elevated at 8,500 U/L. No cardiac arrhythmias noted on examination. Which diagnosis best explains these findings?