A 20-year-old college student presents with a slowly enlarging, non-tender, soft, and fluctuant mass in the left lateral neck. On examination, the mass is mobile and does not move with swallowing or tongue protrusion. Which of the following is the most characteristic internal content of an uncomplicated branchial cleft cyst?

Explanation

Uncomplicated branchial cleft cysts are typically painless, soft, fluctuant, and mobile. They do not move with swallowing or tongue protrusion (differentiating them from thyroglossal duct cysts). While some cystic lesions can transilluminate (e.g., cystic hygroma), it's not a defining characteristic of a branchial cyst. The internal content of a branchial cyst is typically mucoid, yellowish, or cheesy material, often containing cholesterol crystals and desquamated epithelial cells, which is a characteristic histological finding.