Which peripheral blood smear finding is most characteristic of acute promyelocytic leukemia (APL, AML-M3)?

Explanation

## Acute Promyelocytic Leukemia (APL): Pathognomonic Smear Findings **Key Point:** Acute promyelocytic leukemia (AML-M3) is characterized by abnormal promyelocytes containing abundant azurophilic granules and multiple Auer rods. The hallmark finding is **faggot cells** — bundles of Auer rods arranged in parallel within a single cell. ### Morphological Features of APL Blasts | Feature | Description | |---------|-------------| | **Cell type** | Abnormal promyelocytes (M3 FAB classification) | | **Auer rods** | Multiple (often >20 per cell) | | **Auer rod arrangement** | Bundles in parallel ("faggot cells" or "bundle of sticks") | | **Azurophilic granules** | Abundant, coarse, dark-staining | | **Cytoplasm** | Heavily granulated, may obscure nucleus | | **Frequency** | Present in ~80% of APL cases | ### Cytochemical Staining in APL - **Myeloperoxidase (MPO):** Strongly positive (due to abundant granules) - **PAS:** Negative or weakly positive - **Sudan Black:** Strongly positive **Clinical Pearl:** APL is associated with **disseminated intravascular coagulation (DIC)** due to release of procoagulant substances (tissue factor, cancer procoagulant) from leukemic cells. This makes APL a medical emergency requiring immediate treatment with all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). **High-Yield:** The combination of **faggot cells + DIC + t(15;17) translocation** is pathognomonic for APL. The t(15;17) translocation fuses the PML gene on chromosome 15 with the RARA gene on chromosome 17, producing the PML-RARA fusion protein that responds to ATRA-induced differentiation. **Mnemonic: FAGGOT** — **F**ascicles of Auer rods, **A**bundant granules, **G**ranular cytoplasm, **G**ranule-rich promyelocytes, **O**verload of coagulation factors (DIC), **T**(15;17) translocation