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 hereditary cause of mental retardation \nSelect and explain. \nA. Fragile X syndrome, B. Down's syndrome, C. Frederich's ataxia, D. Edward syndrome", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Fragile X-syndrome is the most common "hereditary" cause of mental retardation.\nOvarall most common cause : "Down\'s syndrome".', 'role': 'assistant'}]