**Core Concept**
Cystic hygroma is a congenital anomaly characterized by the abnormal development of lymphatic vessels, leading to fluid-filled cavities or cysts in the neck, chest, or abdomen. The association of cystic hygroma with other syndromes is crucial for prenatal diagnosis and postnatal management.

**Why the Correct Answer is Right**
Turner syndrome is a genetic disorder affecting females, characterized by the absence or abnormality of one X chromosome. Cystic hygroma is a common association with Turner syndrome, particularly in the first trimester, due to the abnormal development of lymphatic vessels and the presence of excess lymph fluid. The combination of cystic hygroma and Turner syndrome has been reported in up to 20% of cases.

**Why Each Wrong Option is Incorrect**
* **Option A:** Trisomy 21 (Down syndrome) is not typically associated with cystic hygroma in the first trimester.
* **Option B:** Trisomy 18 (Edwards syndrome) may have various congenital anomalies, but cystic hygroma is not a characteristic feature.
* **Option D:** Klinefelter syndrome is a genetic disorder in males, characterized by an extra X chromosome, but it is not commonly associated with cystic hygroma.

**Clinical Pearl / High-Yield Fact**
Cystic hygroma is a significant marker for Turner syndrome in the first trimester, highlighting the importance of genetic counseling and detailed fetal assessment.

**Correct Answer:** C. Turner syndrome.