A 32-year-old woman from Mumbai presents with a 3-week history of progressive fatigue, fever, and spontaneous bruising. On examination, she is pale with petechiae over her lower extremities. Laboratory findings show: Hemoglobin 7.2 g/dL, WBC 85,000/μL with 78% blasts, platelets 18,000/μL. Bone marrow aspirate reveals >90% blasts with Auer rods. Flow cytometry shows blasts positive for MPO, CD13, CD33, and negative for CD19. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Promyelocytic Leukemia (APL) ### Clinical Presentation The patient presents with classic features of acute leukemia: fatigue, fever, and hemorrhagic manifestations (petechiae, bruising). The severe thrombocytopenia (18,000/μL) is particularly notable and is a hallmark of APL. ### Key Diagnostic Features **Key Point:** Auer rods in bone marrow aspirate are pathognomonic for acute myeloid leukemia (AML) and are present in ~80% of APL cases. The presence of Auer rods immediately excludes acute lymphoblastic leukemia (ALL). **High-Yield:** APL (AML-M3) is characterized by abnormal promyelocytes with abundant Auer rods (often multiple, called "bundles" or "faggot cells"). The morphology combined with cytochemical positivity for MPO confirms myeloid differentiation. ### Flow Cytometry Interpretation | Feature | APL | AML (other) | ALL | | --- | --- | --- | --- | | MPO | Positive | Positive | Negative | | CD13/CD33 | Positive | Positive | Negative | | CD19 | Negative | Negative | Positive | | HLA-DR | Negative/dim | Positive | Positive | **Key Point:** The myeloid markers (MPO, CD13, CD33) with CD19 negativity confirm myeloid lineage and exclude ALL. ### Cytogenetics & Molecular Biology **High-Yield:** APL is defined by t(15;17)(q24;q21) translocation, which fuses the PML gene with the RARA gene (retinoic acid receptor alpha). This translocation is present in >95% of APL cases and is essential for diagnosis and treatment planning. **Clinical Pearl:** APL is the most curable acute leukemia when treated with all-trans retinoic acid (ATRA) and arsenic trioxide (ATO), with cure rates >90%. Early recognition is critical because untreated APL carries a high risk of disseminated intravascular coagulation (DIC) due to release of procoagulant substances from abnormal promyelocytes. ### Why APL Over Other AML Subtypes? While other AML subtypes (t(8;21), t(15;15), inv(16)) also present with high WBC counts and Auer rods, the **severe thrombocytopenia with hemorrhagic manifestations** and the **morphologic appearance of abnormal promyelocytes with abundant Auer rods** are most consistent with APL. The t(8;21) AML typically presents with better platelet counts and different morphology (blasts with Auer rods but not the characteristic promyelocytic appearance).