This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetic Approach to Pediatric Medicine, Subject name: Pediatrics
[{'content': "Most common cause of Down's syndrome is - \nSelect and explain. \nA. Monosomy 21, B. Trisomy 21, C. Robertsonian translocation, D. Mosaicism", 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Trisomy 21 o Has been explained in previous sessions", 'role': 'assistant'}]