Which reactions are characteristically positive in ALL but negative in AML?

The correct reaction that is characteristically positive in all but negative in acute myeloid leukemia (AML) is the presence of terminal deoxynucleotidyl transferase (TdT) and PAS (Periodic Acid-Schiff) staining. In hematological contexts, TdT is an enzyme that is typically associated with lymphoid progenitor cells; it is a marker used to identify acute lymphoblastic leukemia (ALL). Consequently, TdT positivity is omnipresent in cases of ALL due to the nature of such lymphoblasts, while in AML, TdT is generally absent, making it an important distinguishing feature.

Additionally, PAS staining plays a role in identifying certain leukemias as well. It is characteristically positive in ALL because the leukemic cells often display a high activity of PAS due to their metabolic processes, whereas in AML, the staining tends to be negative or much less pronounced. Thus, the combination of both TdT and PAS positivity serves as a critical marker for differentiating between ALL and AML, reinforcing the diagnosis of ALL when observed together.

The other options are associated with different types of myeloid and lymphoid leukemias and their respective staining characteristics. For instance, some myeloid leukemias may show