A 6-year-old boy presents with fatigue, fever, and night sweats. Physical examination reveals marked pallor. Palpation of his sternum demonstrates diffuse tenderness. Laboratory studies disclose anemia, thrombocytopenia, and leukocytosis. The WBC differential count shows that 90% blasts. A bone marrow biopsy stained immunohistochemically for terminal deoxynucleotidy! transferase (TdT). Further investigation reveals the presence of cytoplasmic u and light chain gene rearrangement. Which of the following is the appropriate stage of development of the cells from which the clone arises? • Pre-B cell Activated blast B cell Pro-B cell Lymphoid stem cells Immature B cell • Mature B cell.

A 6-year-old boy presents with fatigue, fever, and night sweats. Physical examination reveals marked pallor. Palpation of his sternum demonstrates diffuse tenderness. Laboratory studies disclose anemia, thrombocytopenia, and leukocytosis. The WBC differential count shows that 90% blasts. A bone marrow biopsy stained immunohistochemically for terminal deoxynucleotidy! transferase (TdT). Further investigation reveals the presence of cytoplasmic u and light chain gene rearrangement. Which of the following is the appropriate stage of development of the cells from which the clone arises? • Pre-B cell Activated blast B cell Pro-B cell Lymphoid stem cells Immature B cell • Mature B cell.