A 5-year-old child presents with a painless, soft, mobile swelling in the midline of the neck, just below the hyoid bone. The swelling moves upwards on protrusion of the tongue. What is the most likely diagnosis?

Explanation

The classic presentation of a thyroglossal duct cyst (TGC) is a painless, soft, mobile midline neck swelling, often located at or just below the hyoid bone. The pathognomonic sign is its upward movement with tongue protrusion, due to its embryological connection to the foramen cecum at the base of the tongue, often passing through or intimately associated with the hyoid bone. Branchial cleft cysts are typically lateral neck swellings. Cystic hygromas are usually soft, compressible, transilluminating masses, often in the posterior triangle of the neck or axilla. Lipomas are soft, mobile, non-tender masses that do not typically move with tongue protrusion.