Screening test used in first trimester for aneuploidy –
**Core Concept**
The first trimester screening test for aneuploidy involves measuring the levels of two specific serum markers, PAPP-A and beta HCG, in pregnant women. This test is used to assess the risk of chromosomal abnormalities such as Down syndrome, trisomy 13, and trisomy 18.
**Why the Correct Answer is Right**
PAPP-A (Pregnancy-Associated Plasma Protein-A) is a protein produced by the placenta, and its levels are typically lower in pregnancies affected by aneuploidy. Beta HCG (human chorionic gonadotropin) is a hormone produced by the placenta, and its levels are typically higher in pregnancies affected by aneuploidy. The combination of low PAPP-A and high beta HCG levels is associated with an increased risk of aneuploidy. This screening test is based on the principle that these two markers have opposite trends in aneuploid pregnancies, allowing for a more accurate assessment of risk.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because estradiol is not a marker used in the first trimester screening test for aneuploidy. Estradiol is a hormone involved in the regulation of the menstrual cycle and is not directly related to aneuploidy screening.
**Option B:** Incorrect because AFP (alpha-fetoprotein) is a marker used in the second trimester screening test for aneuploidy, not the first trimester. AFP is a protein produced by the fetus, and its levels are typically higher in pregnancies affected by aneuploidy.
**Option D:** Incorrect because inhibin is not a marker used in the first trimester screening test for aneuploidy. Inhibin is a hormone produced by the placenta, but it is not used in combination with beta HCG for aneuploidy screening.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the first trimester screening test for aneuploidy is based on the combination of low PAPP-A and high beta HCG levels. This test is not diagnostic but rather a screening tool that identifies pregnancies at higher risk of aneuploidy, requiring further testing and evaluation.
**✓ Correct Answer: C. PAPP-A & beta HCG**