A 10-year-old boy presents with a soft, non-tender, compressible mass located in the left supraclavicular region, which transilluminates brightly. The mass has been present since birth and slowly increased in size. Which of the following is the most likely diagnosis?

Explanation

The key features here are the supraclavicular location, presence since birth, slow growth, softness, compressibility, and bright transillumination. These are classic characteristics of a cystic hygroma (lymphatic malformation), which is a congenital malformation of the lymphatic system. A second branchial cleft cyst is typically located more anteriorly along the SCM and usually does not transilluminate brightly. A thyroglossal duct cyst is midline. Cervical lymphadenitis would be tender, firm, and not transilluminate.