an 8-year-old boy comes to his physician with a painless and smooth mass located in the midline of his neck at the level of the hyoid bone, as noted by the arrow in the given photo. this palpable, midline neck mass was asymptomatic, but due to recent expansion, it has caused difficulty and pain when swallowing. when he swallows or protrudes his tongue, the mass moves superiorly. what is the most likely diagnosis?