A child presents with a midline swelling beneath the arch of the hyoid bone. What might this swelling be?

The child most likely has a thyroglossal cyst that results from incomplete regression of the thyroglossal duct. These cysts may form anywhere along the line of descent of the thyroid gland as it migrates from the region of the foramen cecum of the tongue to its position in the neck. A cyst must be differentiated from ectopic glandular tissue, which may also remain along this pathway. Ref: Langman's embryology 11th edition Chapter 16.