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
Down syndrome is 
Select and explain. 
A. Trisomy of 21, B. Trisomy of 18, C. Monosomy of 18, D. Monosomy of 21
A: Trisomy of 21
Answer: A: Trisomy of 21 
Explanation: Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46