## **Core Concept**
The question tests knowledge of neck masses, specifically those that are cystic in nature and have characteristic imaging features. The clinical presentation and imaging findings are crucial for diagnosing the condition. The likely diagnosis involves a congenital anomaly that presents as a neck mass.

## **Why the Correct Answer is Right**
The correct answer, **B. Branchial cyst**, is supported by the clinical presentation and imaging findings. The mass is located anterior and medial to the sternocleidomastoid muscle, which is a typical location for branchial cysts. These cysts are congenital anomalies that arise from the branchial apparatus and often present in young adults. They can increase in size and become tender during upper respiratory infections due to secondary infection or inflammation. The ultrasound (USG) and contrast-enhanced computed tomography (CECT) scan findings of a single, round, cystic mass with uniform low echogenicity, lack of internal septations, and smooth rim enhancement are characteristic of a branchial cyst.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Thyroglossal duct cyst is typically located in the midline of the neck, which does not match the description of being just anterior and medial to the right sternocleidomastoid.
- **Option C:** Cystic hygroma (or lymphangioma) can present as a neck mass and may have similar imaging characteristics, but it is more commonly associated with Turner syndrome or as part of a lymphatic malformation and often presents in childhood.
- **Option D:** Epidermoid cyst can present as a neck mass but is usually not as large or as specifically located as described; it also does not typically increase in size with upper respiratory infections.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that branchial cysts are often diagnosed based on their characteristic location (along the anterior border of the sternocleidomastoid muscle), imaging features (cystic with smooth rim enhancement), and clinical behavior (can become larger and tender with infections). They are usually managed with surgical excision, especially if symptomatic or if there's a suspicion of malignancy.

## **Correct Answer: B. Branchial cyst**