Ans. a (Monocytic AML). (Ref. Robbins, Pathology, 8th ed. Table 12-11A ; 7th ed.,692)Revised FAB Classification of Acute Myelogenous Leukemias (AML)CLASSDEFINITIONINCIDENCE (%)MORPHOLOGY/COMMENTSM0Minimally differentiated AML2-3Blasts lack Auer rods and myeloperoxidase but express myeloid lineage surface markers.M1AML without maturation20Some blasts (>3%) are myeloperoxidase positive; few granules or Auer rods and very little maturation beyond the myelo- blast stage of differentiation.M2AML with maturation30-40>20% of marrow cells are myeloblasts, but many cells are seen at later stages of granulocyte differentiation;Auer rods are usually present; often associated with t(8;21).M3Acute pro- myelocytic leukemia5-10Most cells are abnormal promyelocytes, often containing many Auer rods per cell; patients are younger on average (median age 35-40yr); high incidence of DIC; strongly associated with t(15;17).M4Acute myelomono- cytic leukemia15-20Myelocytic and monocytic differentiation evident by cytochemical stains; monoblasts are positive for nonspecific esterase; myeloid cells show a range of maturation; variable numbers of Auer rods; subset associated with inv(16).M5Acute monocytic leukemia10Monoblasts and immature monocytic cells (myeloperoxidase negative, nonspecific esterase positive) predominate;Auer rods are usually absent;older patients;more likely to be associated with organomegaly,adenopathy, and tissue infiltration; the M5b subtype is defined by the predominance of mature-appearing monocytes in the peripheral blood, whereas only immature cells are seen in the M5a subtype.M6Acute erythro leukemia5Most commonly associated with abundant dysplastic erythroid progenitors; >20% of cells of the marrow nonerythroid cells are myeloblasts, which may contain Auer rods; usually occurs in advanced age or following exposure to mutagens (e.g., chemotherapy).M7Acute megakaryo- cytic leukemia1Blasts of megakaryocytic lineage predominate, as judged by expression of platelet-specific antigens;myelofibrosis or increased marrow reticulin often present;Auer rods are absent.