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 syndrome is \nSelect and explain. \nA. Trisomy of 21, B. Trisomy of 18, C. Monosomy of 18, D. Monosomy of 21', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]