**Core Concept**
Increased Nuchal Translucency (NT) is a sonographic marker associated with an increased risk of chromosomal abnormalities, particularly Down syndrome (Trisomy 21), in fetuses between 11 and 14 weeks of gestation. NT is defined as the thickness of the fluid-filled space at the back of the fetal neck.

**Why the Correct Answer is Right**
The pathophysiology behind increased NT is not fully understood. However, it is believed to be related to altered fetal development and fluid dynamics. Some studies suggest that increased NT may be associated with decreased fetal nuchal fluid volume, increased fetal soft tissue, or both. The risk of chromosomal abnormalities increases as the NT measurement exceeds 3.5 mm.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because this option does not provide a valid association with increased NT. While thyroid abnormalities can affect fetal development, they are not directly linked to increased NT.
* **Option B:** Incorrect because this option is unrelated to the pathophysiology of increased NT. While maternal age is a risk factor for chromosomal abnormalities, it is not a direct cause of increased NT.
* **Option C:** Incorrect because this option is not a recognized association with increased NT. While certain genetic syndromes may present with increased NT, this option is not a valid choice.

**Clinical Pearl / High-Yield Fact**
Remember that an increased NT measurement is not a definitive diagnosis of Down syndrome or other chromosomal abnormalities. It is essential to consider NT in conjunction with other ultrasound markers, maternal age, and serum screening results to determine the overall risk.

**Correct Answer: D. Trisomy 21 (Down syndrome)**