This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thyroid disorders, Subject name: Pediatrics
Most common preventable cause of mental retardation:- 
Select and explain. 
A. Down's syndrome, B. Congenital hypothyroidism, C. Rett's syndrome, D. Fragile x syndrome
B: Congenital hypothyroidism
Answer: B: Congenital hypothyroidism 
Explanation: - Congenital hypothyroidism is the M.C preventable cause of mental retardation - In India incidence is 1:1000 - Most common cause of congenital hypothyroidism is thyroid dysgenesis. - Most common cause of congenital hypothyroidism with goitre- Thyroid dyshormonogenesis.