A 4-year-old boy from Delhi presents with a 3-week history of progressive pallor, easy bruising, and fever. On examination, he is pale, has petechiae on the lower limbs, and hepatomegaly (3 cm below costal margin). Laboratory findings: Hemoglobin 7.2 g/dL, WBC 85,000/µL (80% blasts), platelets 35,000/µL. Bone marrow aspirate shows >90% lymphoblasts with fine chromatin and prominent nucleoli. Cytochemistry: MPO negative, PAS positive (block pattern). Which is the most likely diagnosis?

Explanation

## Diagnosis: Acute Lymphoblastic Leukemia (ALL), B-Cell Precursor Type ### Clinical Presentation **Key Point:** ALL is the most common childhood malignancy, accounting for ~80% of pediatric leukemias. The classic triad of pallor, bleeding/bruising, and fever reflects bone marrow failure and leukostasis. ### Morphologic & Cytochemical Features | Feature | ALL | AML | APL | |---------|-----|-----|-----| | **Blast morphology** | Fine chromatin, prominent nucleoli, scant cytoplasm | Abundant cytoplasm, Auer rods possible | Abnormal promyelocytes, Auer rods (bundles) | | **MPO stain** | Negative | Positive | Positive | | **PAS stain** | Positive (block/granular pattern) | Negative/weak | Negative | | **Age of peak incidence** | 2–5 years | Older children/adolescents | Rare in children | **High-Yield:** PAS-positive, MPO-negative blasts with fine nuclear chromatin = ALL. The block-pattern PAS positivity (coarse granules) is pathognomonic for lymphoblasts. ### Immunophenotyping Context The clinical picture (young age, massive lymphadenopathy, hepatomegaly, high WBC) and morphology are consistent with B-cell precursor ALL (BCP-ALL), which comprises ~85% of childhood ALL cases. Expression of CD19, CD10 (common ALL antigen), and TdT would confirm the diagnosis. **Clinical Pearl:** Hepatomegaly and splenomegaly are more prominent in ALL than in AML; mediastinal mass is more typical of T-ALL. ### Why This Is the Best Answer The combination of: 1. Age (4 years = peak incidence for ALL) 2. MPO-negative, PAS-positive blasts 3. Fine chromatin and prominent nucleoli (lymphoid morphology) 4. Massive hepatomegaly and high WBC count ...all point definitively to ALL. [cite:Robbins 10e Ch 13; Kasper et al. Harrison 21e Ch 110]