**Core Concept**
Choroid plexus cysts are a common finding on second-trimester ultrasound, but they are not a specific marker for Down syndrome. The presence of choroid plexus cysts can be seen in up to 2-3% of normal fetuses, and their presence does not necessarily indicate an increased risk of aneuploidy.

**Why the Correct Answer is Right**
Choroid plexus cysts are a normal variant in fetal development, and their presence is not associated with an increased risk of Down syndrome. They are thought to be the result of abnormal cyst formation within the choroid plexus, which is a structure responsible for producing cerebrospinal fluid. The presence of choroid plexus cysts can be seen in both normal and abnormal fetuses, and their absence is not a guarantee of a normal karyotype.

**Why Each Wrong Option is Incorrect**
* **Option A:** A single umbilical artery is a known marker for Down syndrome, as well as other chromosomal abnormalities. The presence of a single umbilical artery is thought to be related to altered blood flow and vasculogenesis during embryonic development.
* **Option C:** Diaphragmatic hernia is a marker for Edwards syndrome (trisomy 18), not Down syndrome. However, it is worth noting that diaphragmatic hernia can be associated with other chromosomal abnormalities, including Down syndrome.
* **Option D:** Duodenal atresia is a known marker for Down syndrome, as well as other chromosomal abnormalities. The presence of duodenal atresia is thought to be related to altered intestinal development and morphogenesis during embryonic development.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the presence of choroid plexus cysts on second-trimester ultrasound does not necessarily indicate an increased risk of Down syndrome. However, the presence of other ultrasound markers, such as a single umbilical artery or duodenal atresia, should raise suspicion for aneuploidy and prompt further evaluation.

**✓ Correct Answer: B. Choroid plexus cyst**