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
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: None