## **Core Concept**
Down's syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Diagnostic serum markers are substances in the blood that can indicate the presence of this condition. These markers are used in screening tests during pregnancy to assess the risk of Down's syndrome in the fetus.

## **Why the Correct Answer is Right**
The correct answer, **Free Estriol**, is a marker that is actually decreased in Down's syndrome. The other options - **Maternal serum alpha-fetoprotein (MSAFP)**, **Human chorionic gonadotropin (hCG)**, and **Inhibin A** - are all markers that have been associated with an increased risk of Down's syndrome when their levels are abnormal. Specifically, in Down's syndrome, MSAFP is typically decreased, hCG is increased, and Inhibin A is increased.

## **Why Each Wrong Option is Incorrect**
- **Option A: Maternal serum alpha-fetoprotein (MSAFP)**: This is indeed a marker used in the screening for Down's syndrome. Low levels of MSAFP are associated with an increased risk of Down's syndrome.
- **Option B: Human chorionic gonadotropin (hCG)**: Elevated levels of hCG are associated with Down's syndrome, making it a relevant marker.
- **Option D: Inhibin A**: This is also a marker that is elevated in Down's syndrome, used in conjunction with others for screening.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the **quadruple screen** (also known as the quad screen) includes MSAFP, hCG, Inhibin A, and **unconjugated estriol (uE3)**, not free estriol specifically, though the terms can be used somewhat interchangeably in this context. This test is used to screen for Down syndrome and other chromosomal abnormalities. Free estriol or unconjugated estriol levels are decreased in Down's syndrome.

## **Correct Answer:** . Free Estriol