A 24-year-old patient consulted a doctor about enlarged submandibular lymph nodes. Objectively: submandibular, axillary and inguinal lymph nodes are enlarged. Chest radiograph shows enlarged mediastinal lymph nodes. In blood: RBCs - 3,4 × 1012/l, Hb- 100 g/l, colour index - 0,88, thrombocytes - 190 × 104 * * * * 9/l, WBCs - 7,5 × 109/l, eosinophils -8%, stab neutrophils - 2%, segmented neutrophils - 67%, lymphocytes - 23%, ESR - 22 mm/h. What study is required to verify the cause of lymphadeno