A 7-year-old boy presents with a lump in his right axilla for 1 month. This has gradually increased in size over time, but is neither painful nor tender.

Around 1 week after the onset of symptoms, he was shown to a primary care physician, who suspected pyogenic lymphadenitis and prescribed a course of cephalexin, without relief.

His medical and surgical histories are unremarkable and all immunizations are up to date. There is no history of exposure to tuberculosis, and he has not come into contact with cats recently.

A complete blood count, C-reactive protein assay, ESR, and renal and liver profiles are all normal.