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: Most common cause of Down syndrome (trisomy) is meiotic nondisjunction. Since maternal age is a significant factor for the incidence of Down's, it also suggests that in most cases the nondisjunction takes place in the ovum (Maternal Nondisjunction). Ref: Robbin's Basic Pathology, 7th Edition, Page 230; Essentials of Human Disease By Leonard Crowley, Page 112.", 'role': 'assistant'}]