A patient presents with severe anemia (Hgb 62 g/L), macrocytosis (MCV 108 fL), thrombocytopenia (PLT 55 x 10^9/L), a hypercellular marrow with erythroid precursors, and testing showing Periodic Acid Schiff POS and Myeloperoxidase POS. The peripheral smear shows myeloblasts. Which diagnosis is most consistent?

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Dual-lineage malignant proliferation involving both erythroid and myeloid components is the hallmark here. The patient’s severe anemia with macrocytosis and thrombocytopenia fits a marrow space that's failing normal hematopoiesis. A hypercellular marrow with prominent erythroid precursors shows an erythroid expansion, while the presence of myeloblasts on the smear reveals an accompanying myeloid leukemic component. The staining pattern supports this combined lineage: erythroid elements that are periodic acid Schiff–positive and myeloblasts that are myeloperoxidase–positive demonstrate two malignant hematopoietic lines rather than a single lineage being neoplastic. Taken together, these features define erythroleukemia (FAB M6), where both erythroid and myeloid lineages proliferate.

In other conditions, you wouldn’t expect this same combination. AML with monocytic differentiation centers on monocytic blasts rather than a prominent erythroid expansion; myelodysplastic syndrome with excess blasts lacks the clear dual-lineage neoplasia and typically shows dysplasia with fewer blasts; acute lymphoblastic leukemia would involve lymphoid blasts and usually lack MPO positivity and the distinct erythroid proliferation pattern seen here.