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 of the following is a preventable cause of mental retardation – 
Select and explain. 
A. Hypothyroidism, B. Down syndrome, C. Cerebral palsy, D. All of the above
A: Hypothyroidism
Answer: A: Hypothyroidism 
Explanation: None