A 5-year-old child has body temperature increased up to febrile numbers, suffers from inertness, weakness. Examination revealed hemorrhage on the skin of limbs and torso. Enlargement of cervical and axillary lymph nodes can be detected. The liver is 4 cm below the costal arch; the spleen is 6 cm below the costal arch. Blood test: erythrocytes - 2,3 × 1012/l, Hb- 60 g/l, platelets - 40 × 109/l, leukocytes - 32,8 × 109/l, eosinophils - 1%, band neutrophils - 1%, segmented neutrophils - 12%, lymphocytes - 46%, monocytes -1%, blasts - 40%, Duke's bleeding time is 9 min. What examination is necessary to make the diagnosis?