**Core Concept**
The first trimester screening for Down's syndrome involves a combination of ultrasound markers and biochemical tests to assess the risk of chromosomal abnormalities. This screening aims to identify pregnancies at higher risk, allowing for further diagnostic testing if needed.

**Why the Correct Answer is Right**
The correct answer involves the measurement of nuchal translucency (NT), a fluid-filled space at the back of the fetal neck. Ultrasound assessment of NT between 11-14 weeks of gestation is a critical component of first trimester screening for Down's syndrome. An increased NT thickness is associated with an elevated risk of aneuploidy, including Down's syndrome. The measurement is typically performed using a transvaginal or transabdominal ultrasound, and the results are combined with other markers, such as maternal age and serum markers like free beta-human chorionic gonadotropin (free β-hCG) and pregnancy-associated plasma protein-A (PAPP-A), to calculate the risk.

**Why Each Wrong Option is Incorrect**
* **Option A:** Maternal serum alpha-fetoprotein (MSAFP) is a serum marker used in second trimester screening for Down's syndrome, not first trimester. Elevated MSAFP levels can indicate an increased risk of aneuploidy, but it is not a primary marker in first trimester screening.
* **Option B:** Fetal echocardiogram is a diagnostic tool used to evaluate fetal heart structure and function, particularly in cases where there is a suspicion of congenital heart disease. While it may be part of a comprehensive fetal assessment, it is not a specific marker for Down's syndrome in first trimester screening.
* **Option C:** Ultrasound assessment of nasal bone absence or hypoplasia is a marker for Down's syndrome, but it is not as sensitive or specific as nuchal translucency measurement. It may be used in conjunction with NT measurement but is not the primary marker.

**Clinical Pearl / High-Yield Fact**
Remember that first trimester screening for Down's syndrome involves a combination of ultrasound markers, including nuchal translucency measurement, and serum markers like free β-hCG and PAPP-A. An increased NT thickness is a critical indicator of an elevated risk, and further diagnostic testing, such as amniocentesis or non-invasive prenatal testing (NIPT), may be recommended based on the screening results.

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