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: Medicine
[{'content': 'Most common trisomy among following is : \nSelect and explain. \nA. 18, B. 21, C. 13, D. 5', 'role': 'user'}, {'content': "Answer: B \nExplanation: Answer is B (Trisomy 21) : The most common abnormalities of chromosome number are `trisomies' - Nelson The most frequent and best known trisomy in humans is trisomy 21 or Down's syndrome. - Nelson This is the commonest chromosomal disorder occurring with a frequency of 1:800 to 1:1000 new borns. Down syndrome occurs more often in offspring of mothers conceiving at older age.", 'role': 'assistant'}]