**Core Concept:** Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. The most effective screening tests for detecting Down's syndrome include:

1. **Question:** Which of the following tests has the highest detection rate for Down's syndrome?
A. Serum biochemistry screening
B. Ultrasound examination
C. Amniocentesis
D. Chorionic villus sampling (CVS)

**Correct Answer:** **D. Chorionic villus sampling (CVS)**

**Why the Correct Answer is Right:** Chorionic villus sampling (CVS) is a prenatal diagnostic test used to detect genetic abnormalities, including Down's syndrome. During CVS, a small sample of placental tissue is taken from the first trimester (11-14 weeks) to analyze the fetal DNA. Since CVS provides direct genetic information, it has a higher detection rate for Down's syndrome compared to other options.

**Why Each Wrong Option is Incorrect:**

A. **Serum biochemistry screening (Option A)**: This test measures specific markers in a woman's blood sample, which can help predict the risk of Down's syndrome but has a lower detection rate compared to CVS.

B. **Ultrasound examination (Option B)**: An ultrasound examination is a valuable tool for assessing fetal growth, detecting structural abnormalities, and estimating the due date. While it can provide information about the fetus and help diagnose some conditions, it is not a diagnostic test for Down's syndrome and has a lower detection rate when compared to CVS.

C. **Amniocentesis (Option C)**: Amniocentesis is a prenatal diagnostic test that involves obtaining a sample of amniotic fluid to analyze the fetal DNA. It is similar to CVS but is usually performed later in pregnancy (15-20 weeks). Although amniocentesis is a reliable test, it has a lower detection rate for Down's syndrome compared to CVS.

**Clinical Pearl:** A combination of screening tests, including both biochemistry and ultrasound, can provide more accurate risk estimates for Down's syndrome. However, the highest detection rate for Down's syndrome is achieved through diagnostic tests like CVS (Option D) and amniocentesis (Option C). In comparison to serum biochemistry screening and ultrasound, CVS has a higher detection rate for Down's syndrome, making it the correct answer.