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
X-linked disease leading to mental retardation is - 
Select and explain. 
A. Myotonic dystrophy, B. Fragile-X syndrome, C. Tuberous sclerosis, D. Phenylketonuria
B: Fragile-X syndrome
Answer: B: Fragile-X syndrome 
Explanation: None