A 25-year-old male presents with a painless, soft, fluctuant mass in the left lateral aspect of his neck, anterior to the sternocleidomastoid muscle. This mass is most commonly a remnant of which embryological structure?

Explanation

Branchial cysts, particularly those presenting in the classic location (lateral neck, anterior to SCM), are typically remnants of the second branchial cleft failing to obliterate completely. During embryological development, the second branchial arch grows caudally over the third and fourth arches, forming the cervical sinus of His, which normally obliterates. Persistence of this sinus can lead to a cyst or fistula.