**Core Concept**
The question is testing the understanding of first-trimester markers for aneuploidy detection using ultrasound. Aneuploidy refers to the presence of an abnormal number of chromosomes in a cell, which can lead to genetic disorders. In the first trimester, sonologists use various markers to screen for aneuploidy, including nuchal translucency (NT), nasal bone, and tricuspid regurgitation.

**Why the Correct Answer is Right**
The correct answer is **nuchal translucency (NT)**. Nuchal translucency is a sonological indicator of aneuploidy in the first trimester, typically measured between 11 and 14 weeks of gestation. An increased NT is associated with an increased risk of aneuploidy, particularly Down syndrome (trisomy 21). The NT is measured as the thickness of the fluid-filled space between the skin and the soft tissues at the back of the fetal neck. An NT of 3.5 mm or greater is considered abnormal and warrants further evaluation.

**Why Each Wrong Option is Incorrect**

**Option A:**
This option is incorrect because **abdominal circumference** is not a primary sonological indicator for aneuploidy in the first trimester. While abdominal circumference can be used to assess fetal growth and development, it is not a specific marker for aneuploidy.

**Option B:**
This option is incorrect because **nasal bone** is actually a marker for aneuploidy, but it is not the most commonly used sonological indicator. A missing or hypoplastic nasal bone is associated with an increased risk of aneuploidy, but it is not as sensitive or specific as nuchal translucency.

**Option D:**
This option is incorrect because **tricuspid regurgitation** is a cardiac anomaly that can be associated with aneuploidy, but it is not a primary sonological indicator for aneuploidy in the first trimester. Tricuspid regurgitation is a Doppler finding that can be used to assess fetal cardiac function, but it is not a specific marker for aneuploidy.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that nuchal translucency is a continuous variable, and the risk of aneuploidy increases with increasing NT thickness. A NT of 3.5 mm or greater should be considered abnormal and warrants further evaluation with advanced imaging or genetic testing.

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