The described CT features are most likely representing infected second branchial cleft cyst type II, the most common type of this congenital anomaly. Classically, it is located at the anteromedial border of the sternocleidomastoid muscle, lateral to the carotid space, and at the posterior margin of the submandibular gland, related to the angle of the mandible. The location can be anywhere from the pharyngeal wall to the skin along the course of the second branchial apparatus.