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
Most common trisomy among following is : 
Select and explain. 
A. 18, B. 21, C. 13, D. 5
B: 21
Answer: B: 21 
Explanation: 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.