A 24-year-old patient visited a doctor complaining of enlargement of his submaxillary lymph nodes. Objectively: submaxillary, axillary and inguinal lymph nodes are enlarged. Chest X-ray shows: enlarged lymph nodes of mediastinum. Blood test: erythrocytes - 3,4 × 1012/l, Hb- 100 g/l, blood colour index - 0,88, platelets - 190 × 109/l, leucocytes - 7,5 × 109/l, eosinophils - 8%, band neutrophils - 2%, segmented neutrophils - 67%, lymphocytes - 23%, ESR - 22 mm/hour. What test must be prescribed to verify the cause of lymphadenopathy?