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?