You are answering a USMLE-style medical question. Question: A 65-year-old woman schedules an appointment with her physician for evaluation of weight loss and weakness. The weakness is more noticeable when climbing stairs and combing hair. The weakness improves after brief exercise. She also has bladder incontinence for the past 4 weeks and was given an anticholinesterase agent with no improvement. What is the most likely cause of the symptoms? Options: A) Duchenne muscular dystrophy B) Hypothyroidism C) Myasthenia gravis D) Paraneoplastic syndrome from small cell carcinoma of the lung Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.