**Core Concept**
Trisomy 21, also known as Down syndrome, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Antenatal ultrasound screening is crucial for early detection and diagnosis.

**Why the Correct Answer is Right**
The most sensitive antenatal ultrasound finding of Trisomy 21 is the **nuchal translucency (NT)**, which refers to the thickness of the fluid-filled space at the back of the fetal neck. An increased NT is associated with an increased risk of Trisomy 21. The NT is measured between 11 and 14 weeks of gestation and is considered abnormal if it exceeds 3.5 mm. This is because the increased NT is due to the accumulation of fluid in the fetal tissues, which is a hallmark of Trisomy 21.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the nasal bone absence is a marker for Trisomy 21, it is not as sensitive as the nuchal translucency.
* **Option B:** The short long bone is a marker for Trisomy 21, but it is not as sensitive as the nuchal translucency.
* **Option D:** The absent ductus venosus is a marker for Trisomy 21, but it is not as sensitive as the nuchal translucency.

**Clinical Pearl / High-Yield Fact**
The nuchal translucency measurement is performed between 11 and 14 weeks of gestation, and an increased NT is associated with an increased risk of Trisomy 21. This is why it is essential to measure the NT carefully during this period.

**Correct Answer: C. Nuchal translucency**