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: Pediatrics
[{'content': "Down's syndrome most commonly occurs due to – \nSelect and explain. \nA. Reciprocal translocation, B. Nondysjunction in maternal meiosis, C. Translocation defect, D. Nondysjunction in paternal meiosis", 'role': 'user'}, {'content': 'Answer: B \nExplanation: In trisomy 21, the additional number 21 chromosome is derived from the mother in 95% of cases.\nIt is due to nondisjunction of the 21st chromosome at the time of meiosis.\nIn nondisjunction, chromosomal pair fails to separate in the first meiotic division during gametogenesis.', 'role': 'assistant'}]