A 28-year-old woman from Delhi presents with a 3-week history of progressive fatigue, fever, and spontaneous bruising. On examination, she is pale with petechiae and hepatosplenomegaly. Laboratory findings show: Hb 7.2 g/dL, WBC 45,000/μL (70% blasts), platelets 18,000/μL. Bone marrow examination reveals >80% blasts with Auer rods. Flow cytometry shows CD13+, CD33+, MPO+, HLA-DR+. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Promyelocytic Leukemia (APL, AML-M3) ### Clinical Presentation The patient presents with the classic triad of APL: 1. **Severe thrombocytopenia** (18,000/μL) — often <50,000/μL 2. **Coagulopathy** — spontaneous bruising and petechiae 3. **High-risk bleeding** — DIC is the hallmark complication ### Morphologic & Immunophenotypic Features | Feature | APL (M3) | Finding in this case | |---------|----------|----------------------| | **Auer rods** | Pathognomonic; often multiple ("faggot cells") | Present ✓ | | **Blast morphology** | Abnormal promyelocytes with heavy granulation | Consistent with morphology | | **Immunophenotype** | CD13+, CD33+, MPO+, HLA-DR+ (or −) | Matches ✓ | | **Cytogenetics** | t(15;17) — PML-RARA fusion | Gold standard (not given but diagnostic) | | **Coagulopathy** | DIC due to release of tissue factor from leukemic cells | Explains bruising/petechiae | **Key Point:** Auer rods are most abundant and characteristic in APL; multiple Auer rods in a single cell ("faggot cells") are virtually pathognomonic for M3. ### Why This Is APL and Not Other Subtypes **High-Yield:** APL is the **only AML subtype where morphology + cytochemistry + immunophenotype + Auer rods converge** to give a diagnosis. The presence of **multiple Auer rods** is the morphologic smoking gun. ### Clinical Significance **Clinical Pearl:** APL is a **medical emergency** due to: - **DIC** (disseminated intravascular coagulation) — caused by release of procoagulants (tissue factor, cancer procoagulant) from abnormal promyelocytes - **Bleeding risk** — spontaneous intracranial hemorrhage, GI bleed, pulmonary hemorrhage - **Treatment response** — APL is the **most curable acute leukemia** (>90% CR rate) with ATRA (all-trans retinoic acid) + arsenic trioxide; early recognition is life-saving **Mnemonic:** **ATRA** = All-Trans Retinoic Acid (induces differentiation of leukemic promyelocytes); **APL** = Acute Promyelocytic Leukemia (t(15;17), PML-RARA fusion, Auer rods, DIC, excellent prognosis with targeted therapy). ### Cytogenetics (Not Provided But Diagnostic) The **t(15;17) translocation** results in the **PML-RARA fusion gene**, which is: - Diagnostic for APL - Detectable by RT-PCR (most sensitive) - The target of ATRA therapy (ATRA binds RARα and triggers differentiation) [cite:Robbins 10e Ch 13]