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': 'X-linked disease leading to mental retardation is - \nSelect and explain. \nA. Myotonic dystrophy, B. Fragile-X syndrome, C. Tuberous sclerosis, D. Phenylketonuria', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]