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
Condition a/w MR - 
Select and explain. 
A. Trisomy 21, B. Fragile - X, C. Homocystinemia, D. All
D: All
Answer: D: All 
Explanation: Ans. is All