**Question:** Which of the following is the investigation of choice in a pregnant lady at 17 weeks of pregnancy, with past history of delivering a baby with Down syndrome?

A. Amniocentesis
B. Serum screening
C. Chorionic villus sampling
D. Ultrasonography

**Correct Answer:** C. Chorionic Villus Sampling (CVS)

**Core Concept:** Chorionic villi are finger-like projections in the placenta that facilitate nutrient exchange between the maternal and fetal circulations. Chorionic villi sampling (CVS) is a prenatal diagnostic test performed to evaluate fetal chromosomes and genetic material.

**Why the Correct Answer is Right:** CVS is the correct answer because it allows for the direct examination of the fetal chromosomes, offering a higher diagnostic accuracy compared to other options in this scenario. Performed between 10-14 weeks of gestation, CVS involves the collection of chorionic villi from the placenta for karyotyping. This test has a higher success rate and fewer complications than amniocentesis, especially in cases with a higher risk of chromosomal abnormalities, such as a history of delivering a baby with Down syndrome.

**Why Each Wrong Option is Incorrect:**

A. Amniocentesis (Amniotic fluid sampling): Amniocentesis is another prenatal diagnostic test for chromosomal abnormalities, but it has a lower success rate than CVS, especially when the gestational age is advanced, as in this case (17 weeks). Additionally, it is more prone to complications compared to CVS.

B. Serum screening: Serum screening is a non-invasive test that estimates the risk of Down syndrome and other chromosomal abnormalities based on the mother's serum markers. While serum screening is useful for risk assessment, it cannot provide a definitive diagnosis of chromosomal abnormalities.

C. Ultrasonography: Ultrasonography is a valuable tool for fetal anatomy assessment, monitoring fetal growth, and detecting structural anomalies. However, it is not a diagnostic test for chromosomal abnormalities and does not provide information about the fetal chromosomes.

**Clinical Pearl:** In this scenario, the key clinical pearl is the importance of selecting the most appropriate prenatal diagnostic test for high-risk pregnancies, such as those with a history of a previous child with Down syndrome. Early invasive testing (CVS or amniocentesis) is preferred over non-invasive testing (serum screening) due to the advanced gestational age and the increased risk of chromosomal abnormalities. Invasive testing provides a definitive diagnosis, while non-invasive testing provides risk assessment only.