A 5-year-old boy brought to the OPD by his Father. He is having complains of loss of appetite, fatigue, malaise, intermittent low grade fever. He also complains bone pain, bruising, oral gum bleeding. On examination Lymph nodes are palpable, spleen is also palpable. Lab investigation reveals TLC of 9,000/uL with a hemoglobin of 5 g/dL and a platelet count of 9,000/uL. What is the next investigation for diagnosis?