A 4-year-old boy has appeared listless during the past week. He exhibits irritability when his arms or legs are touched. In the past 2 days, large ecchymoses have appeared on the right thigh and left shoulder. CBC shows hemoglobin, 9.3 g/dL; hematocrit, 28.7%; MCV, 96 μm3; platelet count, 45,000/mm3; and WBC count, 13,990/mm3. Examination of the peripheral blood smear shows blasts that lack peroxidase-positive granules, but contain PAS-positive aggregates and stain positively for deoxynucleotidyl transferase negative (TdT−). Flow cytometry shows the phenotype of blasts to be CD19+, CD3−, and sIg−. Which of the following is the most likely diagnosis?