**Core Concept**
The question is testing the knowledge of ultrasound markers associated with an increased risk of **Trisomy 21**, also known as **Down syndrome**, in a fetus. **Trisomy 21** is a genetic disorder caused by the presence of an extra copy of chromosome 21. Ultrasound markers play a crucial role in identifying fetuses at high risk of this condition.

**Why the Correct Answer is Right**
Although the correct answer is not specified, one of the most significant ultrasound markers associated with an increased risk of **Trisomy 21** is the presence of an **increased nuchal translucency (NT)**. The **nuchal translucency** is the sonographic appearance of the subcutaneous tissue between the fetal skin and the soft tissue overlying the cervical spine. An **increased nuchal translucency** is associated with a higher risk of **Trisomy 21** due to the potential for cardiac defects and other congenital anomalies.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific details of option A, it is difficult to provide a precise explanation. However, if option A refers to a marker such as **echogenic intracardiac focus**, while it can be associated with **Trisomy 21**, it is not as strongly associated as some other markers.
**Option B:** Similarly, without specifics, if option B refers to a marker like **shortened femur**, it is indeed associated with **Trisomy 21** but may not represent the greatest increased risk compared to other markers.
**Option C:** If option C refers to **choroid plexus cysts**, these are more commonly associated with **Trisomy 18** rather than **Trisomy 21**.
**Option D:** Without the details of option D, it's challenging to provide a specific reason why it might be incorrect.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that an **increased nuchal translucency** is a significant marker for **Trisomy 21**, and its presence should prompt further diagnostic testing, such as **non-invasive prenatal testing (NIPT)** or **amniocentesis**.

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