## Core Concept
A branchial cyst, also known as a branchial cleft cyst, is a congenital anomaly that arises from the remnants of the branchial apparatus during embryonic development. The branchial apparatus consists of branchial arches, clefts, and pouches that form the structures of the head and neck. Abnormal development of these structures can lead to the formation of cysts, sinuses, or fistulas.

## Why the Correct Answer is Right
The correct answer is related to the developmental origin and characteristics of branchial cysts. Typically, branchial cysts are lined by stratified squamous epithelium and sometimes by ciliated columnar epithelium, reflecting their origin from the ectoderm and endoderm of the branchial clefts. They usually present as painless masses along the anterior border of the sternocleidomastoid muscle.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately describe a characteristic feature of branchial cysts.
- **Option B:** Similarly, this option does not provide a correct statement about branchial cysts.
- **Option C:** This option might provide some information but is not the correct statement regarding branchial cysts.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that branchial cysts often present in the second decade of life, and their diagnosis is primarily clinical, supported by imaging studies like ultrasound and CT scans. A notable feature is that they can become infected, which may lead to their initial presentation.

## Correct Answer Line
**Correct Answer: D.**