<prompt> Which is MC genetic cause of Mental RetardationTuberous sclerosisCri-du-chat syndromeFragile-x-syndromeAngles syndrome  <response>: Most common cause of MR is fragile X syndrome. Fragile X syndrome FXS is a genetic disorder. FXS is caused by changes in a gene that scientists called the fragile X mental retardation 1 FMR1 gene when it was first discovered. The FMR1 gene usually makes a protein called fragile X mental retardation protein FMRP. FMRP is needed for normal brain development. People who have FXS do not make this protein. People who have other 