A 25-year-old patient presents with a painless, fluctuant neck mass located along the anterior border of the sternocleidomastoid muscle. What is the most likely diagnosis, and where is it typically located in relation to surrounding anatomical structures?

Second branchial cleft cyst. It typically presents as a painless, fluctuant mass along the anterior border of the sternocleidomastoid muscle, often lateral to the internal jugular vein and at the level of the carotid bifurcation. Key diagnostic features include its cystic nature on imaging, often with a thin wall and clear fluid. Clinical pearls include its common presentation in young adults and potential for infection. Common pitfalls include mistaking it for lymphadenopathy or other neck masses such as a thyroglossal duct cyst.