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An acute myeloid leukemia (AML) that shows positive alpha-naphthyl acetate esterase activity is typically classified as M4, which is associated with monocytic differentiation. Alpha-naphthyl acetate esterase activity is significant because it indicates the presence of monocytic cells, which are characterized by their ability to express this enzyme.
In the context of AML classification per the French-American-British (FAB) system, M4 specifically refers to acute myelomonocytic leukemia, where there is a combined presence of myeloid and monocytic lineage. In this classification, M2 denotes acute myelogenous leukemia with maturation but is not specifically characterized by esterase activity.
Thus, in this situation, the identification of positive esterase activity points toward the monoblastic component of the leukemia, leading to a diagnosis aligned with M4 classification rather than M2. This understanding helps in recognizing the role of enzyme activity in identifying specific leukemia subtypes.