<prompt> Most common preventable cause of mental retardation:-Downs syndromeCongenital hypothyroidismRetts syndromeFragile x syndrome  <response>: - 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. 