Patient here gives a sho history (acute onset) of development of pancytopenia (fever; weakness and gum bleeding suggest leucopenia; anemia and thrombocytopenia respectively). The presence of 26% blasts in the bone narrow suggests the development of acute leukemia and the presence of Auer rods means that the diagnosis is most likely AML. Pseudo Pelger Huet cells are neutrophils having greater than 2 nuclear lobes and are usually seen in myelodysplastic syndrome. In 10% patients, MDS can give rise to AML. So, the AML in question may have developed from MDS. The chief cell in AML is myeloblast for which the staining is positive for myeloperoxidase. Note: Acid phosphatase is useful for lymphoblasts which are seen in ALL Other options: Toluidine blue stain mast cells red-purple (metachromatic staining) and the background blue (ohochromatic staining). Non-specific esterase: AML M4 ,M5.