This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetic and genetic disorders, 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: In approximately 95% cases down syndrome is due to presence of 3 copies of chromosome 21. Approximately 1% cases with trisomy 21 are mosaics . Another 4% have a translocation that involves chromosome 21. Majority of translocation are fusions at the centromere between chromosome 13,14,15,21, and 22 known as robesonial translocation. Reference : Nelson TB of pediatrics edition 19 pg 402.