This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'The electromyogram (EMG) is least useful for the diagnosis of - \nSelect and explain. \nA. Cerebral palsy, B. Spinal muscular atrophy, C. Charcot - Marie - Tooth disease, D. Myasthenia gravis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]