A 9-year-old boy presents with fever, feeling unwell, and easy bruising on his legs. On examination he is pale, blood pressure 100/60 mm Hg, pulse 100/min, and temperature 37.8degC. His lungs are clear, abdomen is soft with a palpable spleen, and there are petechiae and bruises on his legs. His CBC reveals a hemoglobin of 8.5 g/dL, WBC of 17,000/mL, and platelets of 30,000/mL. A blood film reveals neutropenia and leukemic lymphoblasts. Which of the following is the most appropriate initial diagnostic test to confirm the diagnosis?