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': 'All are features of autistic disorder except - \nSelect and explain. \nA. Mental retardation, B. Absent social smile, C. Onset before 21/2 years, D. Third person hallucination', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Mental retardation is seen in 50 - 75% of patients with autism.\nThere is the absence of social smile and eye to eye contact.\nOnset is before 21/2 years.\nThird person hallucination is characteristic of schizophrenia.', 'role': 'assistant'}]