**Core Concept**
Down's syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Prenatal ultrasound can help identify fetal anomalies associated with Down's syndrome. One of the key sonographic findings is a marker of increased nuchal translucency, which is the thickness of the fluid-filled space at the back of the fetal neck.

**Why the Correct Answer is Right**
Increased nuchal translucency is a significant marker for Down's syndrome. This occurs due to an accumulation of fluid in the nuchal fold, which can be detected as a thickened space (greater than 3.5 mm) on ultrasound. The exact mechanism behind this accumulation is not fully understood, but it is thought to be related to impaired lymphatic drainage and increased vascular permeability in the fetus.

**Why Each Wrong Option is Incorrect**
* **Option A:** Shortened nasal bone is actually associated with aneuploidy, particularly Trisomy 18.
* **Option B:** Increased fetal size is a general marker of fetal macrosomia, not specific to Down's syndrome.
* **Option C:** Polyhydramnios (excessive amniotic fluid) is associated with fetal anomalies such as esophageal atresia, not Down's syndrome.

**Clinical Pearl / High-Yield Fact**
A "double bubble" sign on ultrasound, indicating duodenal atresia, is a common association with Down's syndrome. This occurs due to the failure of the duodenum to develop properly during fetal development.

**Correct Answer: C. Increased nuchal translucency.**