**Core Concept**
Branchial cysts, also known as branchial cleft cysts, are congenital anomalies resulting from incomplete closure of the branchial clefts during embryonic development. They are typically found along the lateral aspect of the neck, often near the anterior border of the sternocleidomastoid muscle.

**Why the Correct Answer is Right**
The correct answer is incorrect because branchial cysts are predominantly found in the lateral neck, not in the lung. They arise from remnants of the second branchial cleft and are usually located near the junction of the upper and middle thirds of the anterior border of the sternocleidomastoid muscle. The majority of branchial cysts occur along the neck, not in the mediastinum or lung.

**Why Each Wrong Option is Incorrect**
**Option A:** Seen in mediastinum - This is incorrect because while branchial cysts can be found in the mediastinum, they are not the most common location. However, mediastinal branchial cysts are a distinct entity and should not be confused with the more common cervical branchial cysts.
**Option C:** Are commonly infected - This is incorrect because branchial cysts are not typically infected, although they can become secondarily infected if they are ruptured or if there is an underlying infection.
**Option D:** Multilocular - This is incorrect because while some branchial cysts can be multilocular, not all branchial cysts are multilocular. The majority of branchial cysts are unilocular.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that branchial cysts are often found in the lateral neck, near the anterior border of the sternocleidomastoid muscle. A classic clinical finding is a painless, fluctuant mass that may be tender if infected.

**✓ Correct Answer: B. 50-70% occur in lung**