**Core Concept**
Cystic hygroma is a congenital malformation of the lymphatic system, characterized by abnormal lymphatic vessels and cysts. It typically presents as a soft, painless, and translucent mass in the neck, often associated with other congenital anomalies. The treatment goal is to completely excise the cystic hygroma to prevent complications such as respiratory distress or cardiac issues.

**Why the Correct Answer is Right**
The treatment of choice for cystic hygroma is surgical excision. This involves complete removal of the cystic hygroma to prevent recurrence and associated complications. Surgical excision is generally considered the most effective treatment option, especially in infants and children. The procedure may involve dissection of the surrounding tissue and lymphatic vessels to ensure complete removal of the cystic hygroma.

**Why Each Wrong Option is Incorrect**
**Option A:** Sclerotherapy is a treatment option for varicose veins, not cystic hygroma. It involves injecting a sclerosing agent to close off abnormal blood vessels, which is not applicable in this case.

**Option B:** Radiotherapy is not a suitable treatment option for cystic hygroma, especially in children. It may cause significant side effects and is generally reserved for malignant conditions.

**Option C:** Conservative management with observation and monitoring may be considered in some cases, but it is not the treatment of choice for cystic hygroma. Complete excision remains the most effective treatment option to prevent complications.

**Clinical Pearl / High-Yield Fact**
Cystic hygroma is often associated with other congenital anomalies, such as chromosomal abnormalities, cardiac defects, or cleft palate. A thorough preoperative evaluation is essential to identify these associated conditions and plan appropriate management.

**Correct Answer: C. Surgical excision.**