<prompt> Which of the following is not a feature of 21st trisomy?Mental retardationDuodenal atresiaHypeonicityCHD  <response>: Downs syndrome 21st trisomy is the most common chromosomal disorder and a major cause of mental retardation. Children with Downs syndrome develop muscle hypotonia not hypeonia. Reference: Essential Paediatrics by O.P Ghai 6e page 592. 