**Core Concept**
Down syndrome, also known as trisomy 21, is a genetic disorder caused by an extra copy of chromosome 21. Early prenatal diagnosis is crucial for managing the condition and planning for the baby's future.

**Why the Correct Answer is Right**
The diagnosis of Down syndrome at 11 weeks is best assessed by **nuchal translucency (NT) measurement**. This involves using ultrasound to measure the thickness of the fluid-filled space at the back of the fetal neck. An increased NT is associated with an increased risk of Down syndrome and other chromosomal abnormalities. The measurement is usually performed between 11 and 14 weeks of gestation, and it is used in conjunction with other markers and tests, such as maternal age and serum screening, to determine the risk of Down syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** **Maternal serum alpha-fetoprotein (MSAFP)** is a screening test that measures the level of alpha-fetoprotein in the mother's blood. While it can be used to screen for Down syndrome, it is not the most accurate or specific test for diagnosing the condition at 11 weeks.

**Option B:** **Ultrasound markers** such as nasal bone absence, ductus venosus flow, and tricuspid regurgitation can be used to screen for Down syndrome, but they are not as sensitive or specific as nuchal translucency measurement at 11 weeks.

**Option C:** **Chorionic villus sampling (CVS)** is a diagnostic test that involves removing a small sample of placental tissue to test for chromosomal abnormalities. While it can be used to diagnose Down syndrome, it is typically performed between 10 and 12 weeks of gestation and involves a more invasive procedure than nuchal translucency measurement.

**Option D:** **Amniocentesis** is a diagnostic test that involves removing a small sample of amniotic fluid to test for chromosomal abnormalities. While it can be used to diagnose Down syndrome, it is typically performed between 15 and 20 weeks of gestation and involves a more invasive procedure than nuchal translucency measurement.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that nuchal translucency measurement should be performed by an experienced sonographer, and the measurement should be taken at the correct gestational age (between 11 and 14 weeks) to achieve the most accurate results.

**Correct Answer: A. Nuchal translucency measurement.**