Final answer:
The quad screen is a prenatal test assessing the risk of Trisomy 21 (Down Syndrome), which shows increased incidence with maternal age and is confirmed by an extra chromosome 21 in the fetus' karyotype.
Step-by-step explanation:
The quad screen is a prenatal test that measures levels of four substances in a pregnant woman’s blood: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), estriol, and inhibin A. These markers are used to assess the risk of the fetus having certain genetic conditions, including Trisomy 21 (Down Syndrome). Data indicates that the incidence of Trisomy 21 increases with maternal age, particularly after the mother reaches the age of 30, with a more dramatic increase after age 35. A karyotype can confirm Trisomy 21 by showing an extra copy of chromosome 21. This condition is the most common trisomy in humans that can lead to a viable birth, although many Trisomy 21 pregnancies result in miscarriage during the first trimester.