<prompt> Which is M.C genetic cause of mental retardation -Tuberous sclerosisFragile-x-syndromeCri-dv-chat syndromeAngels syndrome  <response>: Ans. is b i.e., Fragile-x-syndrome Mental retardation :-IQ 70Mild MR IQ between50-70Moderate IQ between35-50Severe IQ between20-35Profound MR IQ between 20Etiology :-Mild MR IQ 50 usually Environmental factor.Severe MR IQ 50 usually Biologic causes.Most common cause of Mild MR include genetic syndrome writh multiple minor congenital anomalis.In children with severe MR, biological cause Most commonly prenatal can be identified in 75 of causesCauses-include-chromosomal - Down syndromeCauses-include-genetic - Fragilex syndromeMost common genetic cause of mild MR fragile xMost common genetic cause of moderate MR down syndrome and in general population Mild MR Moderate MR so overall M.C. genetic cause of MR is fragile-x-syndrome.Other features of fragile-x-syndromeo See in males femaleso Autistic behaviorMacro orchidismLarge size facial feature.Long faceProminance jawrLearning disability 