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
Which is MC genetic cause of mental retardation - 
Select and explain. 
A. Tuberous sclerosis, B. Cri-du-chat syndrome, C. Fragile-x-syndrome, D. Angel's syndrome
C: Fragile-x-syndrome
Answer: C: Fragile-x-syndrome 
Explanation: Ans. is 'c' i.e., Fragile-X-syndromeo Down's syndrome is the most common genetic cause of mental retardation, and fragile-X is second to Down's.Also know o Most common inherited cause of mental retardation is fragile-X-syndrome (because down's syndrome is congenital cause but not inherited).