AKT-2900

A 27-year-old man presents with a lump on the right side of his neck which he first noticed two months ago. He tried several homeopathic medications but the lump steadily increased in size. He also noticed some shortness of breath and sweating at night. On examination, he has a large mass that is firm, non-tender and not fixed to deeper structures or to the skin. You suspect the mass is lymph nodes. He is slightly pale but no other masses are palpable. His temperature is 38°C.
Which of the following investigations is most likely to be diagnostic?