<prompt> True regarding Down syndrome is?Increased paternal age is a risk factorKaryotyping is not needed in all patients85 of affected patients have 1 more chromosome 21Increased nuchal translucency  <response>: Answer- B. Karyotyping is not needed in all patientsDowns syndrome is the most common chromosomal disorder and most common congenital cause of mental retardation 2ndmost common genetic cause of mental retardation is Fragile -X sydrome.Trisomy 21- There is an extra chromosome 21 which is due to meiotic nondisjunction in ovum.The most impoant risk factor is advanced maternal age 35 Years.Antenatal Screening for Down syndromeFollowing methods are used :-Triple test It includes i Unconjugated estrogen estriol: decreased; ii Maternal serum alphafeto protein MSAFP :decreased; and iii hCG: increasedNew markers: These are i Increased inhibin A in maternal blood; and ii Decreased PAPA pregnancy associated plasma protein.USG: It shows : i Increased nuchal translucency increased nuchal fold thickness; ii Ductus venous flow reversed; and iii Nasal bone hypoplasia. 