A 4-year-old boy from Delhi presents with a 3-week history of fever, pallor, and easy bruising. On examination, he is febrile (38.5°C), has hepatosplenomegaly, and petechial rash on lower limbs. Complete blood count shows hemoglobin 7.2 g/dL, WBC 85,000/μL with 65% blasts, and platelets 35,000/μL. Bone marrow aspiration reveals >90% lymphoblasts with cytoplasmic vacuoles and Auer rods. Flow cytometry is pending. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Myeloid Leukemia (AML) ### Clinical Presentation The vignette presents a 4-year-old with classic acute leukemia features: fever, pallor, easy bruising (thrombocytopenia), hepatosplenomegaly, and petechiae. The acute presentation over 3 weeks is typical of childhood acute leukemias. ### Laboratory Findings — Key Differentiators | Feature | AML | ALL | CML | |---------|-----|-----|-----| | **Auer rods** | Present (pathognomonic) | Absent | Absent | | **Cytoplasmic vacuoles** | Common in AML-M3, M4, M5 | Rare | Absent | | **Blast morphology** | Larger, more cytoplasm, Auer rods | Smaller, scant cytoplasm | Mature myeloid forms | | **Age of presentation** | Peak 0–4 years (AML-M7) and 10–14 years | Peak 2–5 years (75% of ALL) | Rare in children <5 years | | **WBC count** | Variable (20,000–100,000) | Often very high (>100,000) | Usually >100,000 with left shift | **Key Point:** The presence of **Auer rods** is pathognomonic for AML and is the single most discriminating finding. Auer rods are abnormal fusion products of azurophilic granules and are never seen in ALL or CML. ### Morphologic Clues in This Case 1. **Auer rods** — diagnostic for AML 2. **Cytoplasmic vacuoles** — suggest myeloid differentiation (common in AML-M3, M4, M5) 3. **Blast percentage >90%** — consistent with acute leukemia (not CML, which shows myeloid maturation spectrum) **High-Yield:** Auer rods are formed by abnormal aggregation of azurophilic granules in myeloid blasts. They are a hallmark of AML and prompt immediate recognition. ### Prognosis & Next Steps - AML in children has improved outcomes with modern chemotherapy (5-year survival ~60–70% with intensive therapy) - Flow cytometry will confirm myeloid lineage (CD13, CD33 positive; CD19 negative) - Cytochemistry (MPO, Sudan Black) will be positive, confirming myeloid differentiation - Cytogenetics and molecular testing (FLT3-ITD, NPM1, CEBPA) guide risk stratification [cite:Robbins 10e Ch 13]