## **Core Concept**
Antenatal screening for Down syndrome in the general population primarily involves assessing the risk through various markers. The most commonly used method is the combined screening test, which includes maternal age, fetal nuchal translucency (NT) thickness on ultrasound, and maternal serum biochemical markers.

## **Why the Correct Answer is Right**
The correct approach for antenatal screening of Down syndrome involves a combination of **maternal age**, **fetal nuchal translucency (NT) thickness**, and **maternal serum screening (MSS)** for certain biochemical markers such as **free beta-human chorionic gonadotropin (β-hCG)** and **pregnancy-associated plasma protein-A (PAPP-A)**. This combined screening, often performed between 11 and 14 weeks of gestation, provides a risk assessment for Down syndrome. The markers help identify pregnancies at higher risk, which may then undergo diagnostic testing like amniocentesis or chorionic villus sampling (CVS) for confirmation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incomplete and does not specify a recognized screening method for Down syndrome.
- **Option B:** Similarly, this option lacks specificity regarding established screening protocols for Down syndrome.
- **Option C:** This option is also unspecified and does not correspond with standard practices for Down syndrome screening.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **first-trimester screening** (FTS), which includes the measurement of fetal NT and maternal serum levels of β-hCG and PAPP-A, is a widely accepted and effective method for Down syndrome screening. This approach allows for early risk assessment and decision-making regarding further diagnostic testing.

## **Correct Answer:** . Combined screening (maternal age, NT, and serum markers).