This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetics And Genetic Disorders, Subject name: Pediatrics
[{'content': 'A Down syndrome child is mentally retarded. All cytogenetic abnormalities may occur except? \nSelect and explain. \nA. Deleted chromosome 21, B. Trisomy 21, C. Robertsonian translocation, D. Mosaic', 'role': 'user'}, {'content': "Answer: A \nExplanation: a. Deleted chromosome 21(Ref: Nelson's 20/e p 610-615, Ghai 8/e p 637-639)An extra copy of chr 21 is required to cause trisomy 21 (Down syndrome), so deletion of chr 21 will not cause it.", 'role': 'assistant'}]