A 4-year-old boy presents with a 3-week history of progressive pallor, easy bruising, and recurrent fever. His mother reports he has been reluctant to play and complains of bone pain. On examination, he is pale, has petechiae on his lower limbs, hepatomegaly (3 cm below costal margin), and splenomegaly (2 cm below costal margin). Laboratory investigations reveal: Hemoglobin 7.2 g/dL, WBC 85,000/μL (with 70% blasts), platelets 35,000/μL. Bone marrow aspiration shows >90% blasts with high nuclear-to-cytoplasmic ratio and fine chromatin. Flow cytometry is positive for CD19, CD10, and TdT. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: B-ALL, Common Type ### Key Diagnostic Features **Key Point:** This case presents the classic presentation of childhood acute lymphoblastic leukemia (ALL), specifically the common type (c-ALL), which accounts for ~60–70% of pediatric ALL cases. ### Immunophenotype Analysis The flow cytometry panel is diagnostic: - **CD19+** — B-lineage marker (present in all B-ALL) - **CD10+** — common ALL antigen (CALLA); defines "common type" ALL - **TdT+** — terminal deoxynucleotidyl transferase; marker of lymphoid immaturity This immunophenotype (CD19+ CD10+ TdT+) is pathognomonic for B-ALL, common type. ### Clinical Presentation | Feature | Finding | Significance | |---------|---------|---------------| | Age | 4 years | Peak incidence of ALL is 2–5 years | | Presentation | Pallor, bruising, fever, bone pain | Typical for high tumor burden | | Hepatosplenomegaly | Present (both organs) | Leukemic infiltration | | WBC | 85,000/μL | High-risk leukocytosis (>50,000/μL) | | Blasts | >90% in marrow | Diagnostic for acute leukemia | | Morphology | High N:C ratio, fine chromatin | Lymphoblastic, not myeloid | ### Why NOT the Other Options **High-Yield:** AML and APL both have myeloid markers (MPO+, CD13, CD33); this patient is CD19+ TdT+, which is lymphoid. Burkitt lymphoma presents with a mass lesion and different immunophenotype (CD20+ CD10+ but TdT−). ### Prognosis and Risk Stratification **Clinical Pearl:** This patient has several high-risk features: - Age 4 years (favorable, but WBC >50,000 is unfavorable) - High leukocytosis (85,000/μL) → high-risk disease - Hepatosplenomegaly → extramedullary involvement Despite these high-risk features, B-ALL common type has better prognosis than T-ALL or AML in children, with 5-year event-free survival ~85–90% with modern chemotherapy. ### Treatment Approach **Key Point:** Standard induction therapy includes: 1. Vincristine + daunorubicin + L-asparaginase + corticosteroids 2. CNS prophylaxis (intrathecal chemotherapy ± cranial radiation) 3. Consolidation and maintenance phases based on risk stratification [cite:Lanzkowsky Pediatric Hematology-Oncology 6e Ch 10]