Evaluation of a Lump on the Neck.

A 30-year-old male patient presents for evaluation of a lump on his neck. He denies pain, itch, erythema, edema, or any other symptoms. He is ^concerned because it won’t ^ go away. He says, “I noticed it a few months ago, then it seemed to disappear, and now it is  back.” The AGACNP proceeds with a history and physical exam and concludes which of the following as the leading differential diagnosis?