A 72-year-old woman presents with a 3-month history of progressive difficulty rising from a chair, climbing stairs, and combing her hair. She denies any sensory loss, muscle pain, or rash. On examination, there is symmetrical weakness of proximal muscles in both upper and lower limbs (MRC grade 4/5). Distal strength, sensation, and reflexes are normal. Blood tests reveal a markedly elevated Creatine Kinase (CK) level (e.g., > 5000 U/L). What is the most likely diagnosis?