A 3-year-old boy presents with a painless swelling in the lateral neck that increases in size with upper respiratory tract infection. On examination, a smooth, compressible mass is palpated anterior to the sternocleidomastoid muscle. Which is the most common embryological origin of this branchial cyst?

Explanation

## Branchial Cysts: Embryological Classification and Clinical Presentation **Key Point:** Branchial cysts derived from the second pharyngeal pouch account for 95% of all branchial cysts, making them the most common type encountered clinically. ### Classification of Branchial Cysts by Pouch Origin | Pouch Origin | Frequency | Location | Clinical Features | |--------------|-----------|----------|-------------------| | **2nd (Most Common)** | 95% | Anterior to SCM, between angles of jaw | Painless swelling, increases with URTI, may discharge via fistula | | 1st | 1-2% | Angle of mandible, near ear | Often infected, may have otologic symptoms | | 3rd | 2-3% | Lower neck, near thyroid | Deeper location, may present with thyroid symptoms | | 4th | <1% | Very rare, deep neck | Extremely uncommon | **High-Yield:** The classic presentation of a second branchial cyst is a **lateral neck mass in a child that enlarges with upper respiratory infections**. This occurs because the cyst communicates with the pharynx via the tonsillar fossa, allowing infection to track into the cyst during URTI. ### Embryological Basis 1. **Second pharyngeal pouch** gives rise to the tonsillar fossa and palatine tonsil 2. **Incomplete obliteration** of the second pouch during weeks 5–7 of development 3. **Epithelial remnant** persists as a cyst lined with stratified squamous epithelium 4. **Communication** with pharynx via the tonsillar fossa explains the association with URTI **Clinical Pearl:** Second branchial cysts often present with a **branchial fistula** — an external opening in the neck that may drain mucus or pus, typically located along the anterior border of the sternocleidomastoid muscle at the junction of the middle and lower thirds of the neck. **Mnemonic:** **95-2-3-4 Rule** — 95% are 2nd pouch, 2–3% are 3rd, <1% are 4th, and 1–2% are 1st. ### Differential Diagnosis ```mermaid flowchart TD A[Lateral neck mass in child]:::outcome --> B{Location and features?}:::decision B -->|Anterior to SCM, enlarges with URTI| C[2nd branchial cyst]:::outcome B -->|Angle of mandible, near ear| D[1st branchial cyst]:::outcome B -->|Lower neck, near thyroid| E[3rd branchial cyst]:::outcome B -->|Midline, moves with swallowing| F[Thyroglossal duct cyst]:::outcome ```