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: Psychiatry
[{'content': "Most common genetic cause of mental retardation is \nSelect and explain. \nA. Down's syndrome, B. Fragile X syndrome, C. Frederich's ataxia, D. Ataxia telangietasia", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Most common cause of mental retardation is Down syndrome followed by\xa0Fragile X\xa0syndrome.', 'role': 'assistant'}]