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: Pathology
[{'content': "Down's syndrome is most commonly caused by ? \nSelect and explain. \nA. Maternal nondisjunction, B. Paternal nondisjunction, C. Translocation, D. Mosaicism", 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is. 'a' i.e., Maternal nondisjunction o The most common cause of trisomy 21 (Down's syndrome) is maternal nondisjunction in meiosis 1. In most cases the meiotic nondisjunction of chromosome 21 occurs in the ovum and in 95% of the cases the extra chromosome is of maternal origin.", 'role': 'assistant'}]