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: Psychiatry
Most common genetic cause of mental retardation is 
Select and explain. 
A. Down's syndrome, B. Fragile X syndrome, C. Frederich's ataxia, D. Ataxia telangietasia
A: Down's syndrome
Answer: A: Down's syndrome 
Explanation: Most common cause of mental retardation is Down syndrome followed by Fragile X syndrome.