In Downs syndrome, the 2nd trimester quadruple test done at 14-20 weeks measure levels of all of the following, EXCEPT:
**Core Concept**
Down syndrome is a genetic disorder caused by an extra copy of chromosome 21 (trisomy 21). The 2nd trimester quadruple test is a screening test used to assess the risk of Down syndrome in pregnant women. This test measures levels of four substances in the maternal serum: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), unconjugated estriol (uE3), and inhibin-A.
**Why the Correct Answer is Right**
The quadruple test measures levels of AFP, hCG, uE3, and inhibin-A to assess the risk of Down syndrome. The levels of these substances are used in combination to estimate the likelihood of the fetus having Down syndrome. Inhibin-A is a protein produced by the placenta that helps regulate the production of follicle-stimulating hormone (FSH) and is used as a marker for Down syndrome.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the 2nd trimester quadruple test measures levels of AFP, hCG, uE3, and inhibin-A, but not AFP alone. AFP is one of the substances measured, but the question asks for an exception, and AFP is not the correct answer because it is one of the four substances measured.
* **Option B:** This option is incorrect because hCG is one of the substances measured in the quadruple test. Elevated levels of hCG are associated with an increased risk of Down syndrome.
* **Option D:** This option is incorrect because uE3 is one of the substances measured in the quadruple test. Low levels of uE3 are associated with an increased risk of Down syndrome.
**Clinical Pearl / High-Yield Fact**
It's essential to note that the quadruple test is a screening test and not a diagnostic test. A positive result indicates an increased risk of Down syndrome, but further diagnostic testing, such as amniocentesis or chorionic villus sampling (CVS), is necessary to confirm the diagnosis.
**Correct Answer: A.**