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 hereditary cause of mental retardation 
Select and explain. 
A. Fragile X syndrome, B. Down's syndrome, C. Frederich's ataxia, D. Edward syndrome
A: Fragile X syndrome
Answer: A: Fragile X syndrome 
Explanation: Fragile X-syndrome is the most common "hereditary" cause of mental retardation.
Ovarall most common cause : "Down's syndrome".