A 35-year-old man presents with sudden-onset pancytopenia, a WBC of 22,000/µL, and a peripheral smear showing cells with reniform nuclei and prominent azurophilic granules that form bundles. Bone marrow confirms hypercellularity with >20% blasts. The pathognomonic finding on electron microscopy would reveal:
- A Auer rods composed of fused primary granules ✓
- B Birbeck granules in neoplastic cells
- C Pseudo-Chediak–Higashi granules
- D Russell bodies within cytoplasm
Correct answer: A. Auer rods composed of fused primary granules
Explanation
The description — hypergranular blasts with bundled Auer rods (faggot cells), pancytopenia, and DIC — is classic for Acute Promyelocytic Leukemia (APL, AML-M3). Auer rods are crystalline aggregates of fused primary (azurophilic) granules visible on EM and also on light microscopy. Birbeck granules are seen in Langerhans cell histiocytosis. Russell bodies are immunoglobulin aggregates in plasma cells. Pseudo-Chediak–Higashi granules occur in AML but are large single granules, not the defining feature of APL.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.