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: Pediatrics
A Down syndrome child is mentally retarded. All cytogenetic abnormalities may occurs except? 
Select and explain. 
A. Deleted 21, B. Trisomy 21, C. Robesonia translocation, D. Mosaic
A: Deleted 21
Answer: A: Deleted 21 
Explanation: Ans is 'a' i.e., Deleted 21