A 3-year-old child has been admi- tted to a hospital because of ostealgia and body temperature rise up to 39°C. Objectively: the patient is in grave condition, unable to stand for ostealgia, there is apparent intoxication, lymph nodesare enlarged up to 1,5 cm. Liver can be palpated 3 cm below the costal margin, spleen - 2 cm below the costal margin. In blood: RBCs - 3,0- 1012/l, Hb- 87 g/l, colour index - 0,9, thrombocytes - 190 - 109/l, WBCs - 3,2 - 109/l, eosinophils - 1, stab neutrophils - 1, segmented neutrophils -0, lymphocytes - 87, monocytes - 2, ESR - 36 mm/h. What examination should be conducted in order to specify the diagnosis?