<s>[INST] context: These findings are characteristic of a childhood acute lymphoblastic leukemia of the precursor-B-cell type. The rapid expansion of the marrow caused by proliferation of blasts can lead to bone pain and tenderness. Features supporting an acute leukemia are anemia, thrombocytopenia, and the presence of blasts in the peripheral blood and bone marrow. Anemia and thrombocytopenia result from suppression of normal hematopoiesis by the leukemic clone in the marrow. The phenotype of CD19+, CD3-, and sIg- is typical of pre-B cells. TdT is a marker of early T-cell-type and B-cell-type lymphoid cells. An acute myelogenous leukemia is a disease of young to middle-aged adults, and there would be peroxidase positive myeloblasts and phenotypic features of myeloid cells. Chronic lymphocytic leukemia is a disease of older adults; patients have many small circulating mature B lymphocytes. Chronic myelogenous leukemia is a disease of adults, and the WBC count is quite high; the peripheral blood contains some myeloblasts, but other stages of myeloid differentiation also are detected. In idiopathic thrombocytopenic purpura, only the platelet count is reduced because of antibody-mediated destruction of platelets. [/INST] question: 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 mm3; 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? | options: A: Acute lymphoblastic leukemia B: Acute myelogenous leukemia C: Chronic lymphocytic leukemia D: Chronic myelogenous leukemia | answer: A </s>