<prompt> Which is MC genetic cause of mental retardation -Tuberous sclerosisCri-du-chat syndromeFragile-x-syndromeAngels syndrome  <response>: Ans. is c i.e., Fragile-X-syndromeo Downs syndrome is the most common genetic cause of mental retardation, and fragile-X is second to Downs.Also know o Most common inherited cause of mental retardation is fragile-X-syndrome because downs syndrome is congenital cause but not inherited. 