A 48-year-old man presents with marked generalized gingival hypertrophy, mucosal bleeding, and fatigue. Gum biopsy shows diffuse infiltration by large mononuclear cells with abundant finely vacuolated cytoplasm, which stain positive for alpha-naphthyl butyrate esterase and are myeloperoxidase weakly positive. Peripheral blood shows 30% monoblasts. What is the most likely FAB subtype?
- A Acute promyelocytic leukemia, M3
- B Acute monocytic leukemia, M5 ✓
- C AML with maturation, M2
- D Acute megakaryoblastic leukemia, M7
Correct answer: B. Acute monocytic leukemia, M5
Explanation
Extramedullary infiltration of gums and skin by monoblasts is characteristic of acute monocytic leukemia. Monocytic cells are positive for nonspecific esterases such as alpha-naphthyl butyrate esterase, which distinguishes them from granulocytic blasts that are strongly myeloperoxidase positive. M3 blasts contain dense Auer rods and cause DIC, M2 shows granulocytic maturation without tissue infiltration, and M7 expresses platelet glycoproteins such as CD41 and CD61.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.