True regarding Down syndrome is?

Answer- B. Karyotyping is not needed in all patientsDown's 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.