**Core Concept**
In the second trimester, ultrasound markers are used to assess the risk of Down's syndrome, a genetic disorder caused by an extra copy of chromosome 21. These markers include nuchal translucency, nasal bone absence, and ductus venosus flow abnormalities, among others.

**Why the Correct Answer is Right**
Ultrasound markers for Down's syndrome in the second trimester include:
- Nuchal translucency (NT) thickness > 3.5 mm, indicating an increased risk of aneuploidy.
- Absence of the nasal bone, which is a soft tissue marker that can be associated with Down's syndrome.
- Ductus venosus flow abnormalities, such as reversed or absent flow, which can indicate an increased risk of cardiac anomalies and Down's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a marker of Down's syndrome. However, without knowing the actual options, it is difficult to provide a specific explanation.

**Option B:** This option is incorrect because it is not a recognized marker of Down's syndrome. However, without knowing the actual options, it is difficult to provide a specific explanation.

**Option C:** This option is incorrect because it is not a marker of Down's syndrome. However, without knowing the actual options, it is difficult to provide a specific explanation.

**Option D:** This option is incorrect because it is not a marker of Down's syndrome. However, without knowing the actual options, it is difficult to provide a specific explanation.

**Clinical Pearl / High-Yield Fact**
When interpreting second-trimester ultrasound markers, it's essential to remember that a single abnormal marker does not necessarily confirm Down's syndrome. A combination of markers and maternal age should be considered when assessing the risk.

**Correct Answer:** Not enough information to provide a correct answer. Please provide the actual options.