A 62-Year-Old Man With Progressive Limb Weakness, Involuntary Movements, and HyperCKemia

A 62-year-old man had progressive lower extremity weakness and involuntary movements. Muscle magnetic resonance imaging revealed multifocal hyperintense lesions in the posterior thigh muscles, sparing anterior thigh and adductors. Biopsy of the right vastus lateralis showed fiber size variability and occasional internalized nuclei. What is your diagnosis?