**Core Concept**
Aneuploidy refers to abnormal chromosome numbers, commonly seen in fetal development, and is often associated with structural or morphological abnormalities detectable by ultrasound. While ultrasound findings are not direct indicators of chromosomal abnormalities, certain anomalies show a stronger statistical association with aneuploidy such as trisomy 21 (Down syndrome) or trisomy 18 (Edwards syndrome).

**Why the Correct Answer is Right**
Cystic hygroma, a large cystic lesion in the neck region of the fetus, is associated with a significantly increased risk of chromosomal abnormalities, particularly trisomy 13, trisomy 18, and monosomy X. It arises from lymphatic malformation and is linked to impaired lymphatic development, which is more common in aneuploid fetuses. Studies show that the presence of a cystic hygroma increases the likelihood of aneuploidy by 3–5 fold compared to normal fetuses, making it one of the strongest ultrasound predictors among the listed options.

**Why Each Wrong Option is Incorrect**
Option A: Choroid plexus cyst is associated with an increased risk of aneuploidy, especially trisomy 21, but its predictive value is lower than cystic hygroma and is often transient.
Option B: Nuchal translucency is a well-known marker for aneuploidy, particularly trisomy 21, but its association is moderate and primarily used in screening, not as a strong predictor as cystic hygroma.
Option D: Single umbilical artery is a rare anomaly and is more commonly linked to other congenital issues like renal anomalies or vascular defects, not directly or strongly associated with aneuploidy.

**Clinical Pearl / High-Yield Fact**
Cystic hygroma should always be evaluated in the context of fetal aneuploidy, especially in high-risk pregnancies. It is one of the few ultrasound findings that has a strong, direct association with chromosomal abnormalities and is a key indicator for further genetic testing.

✓ Correct Answer: C. Cystic hygroma