Approach to head and neck masses (pediatrics) Quiz: Ace Your Exams

An 8-year-old boy is brought to the clinic for evaluation of a painless swelling on the right side of the neck that has recently become more noticeable. A parent reports that the mass has been present intermittently for several years and enlarges during colds or upper respiratory infections. There is no fever, weight loss, night sweats, or recent trauma, and the child is otherwise healthy and up to date on immunizations. The family has a pet cat at home. On examination, the child is afebrile and in no distress. A 3 cm soft, non-tender, fluctuant mass is noted anterior to the right sternocleidomastoid muscle. The overlying skin appears normal without erythema or warmth. The mass does not move with swallowing or tongue protrusion. Oropharyngeal and lymphatic examinations are unremarkable. A contrast-enhanced CT scan of the neck reveals a well-defined, low-attenuation cystic structure lateral to the carotid sheath and anterior to the sternocleidomastoid muscle. Which of the following is the most likely cause of this condition?