A 6-year-old child presents with pallor, fatigue, generalized petechiae, and bone pain that wakes him at night. Physical exam reveals cervical and axillary lymphadenopathy and splenomegaly palpable 4 cm below the left costal margin. Complete blood count reveals hemoglobin 7.2 g/dL, platelet count 28,000/microL, and white blood cell count 62,000/microL with 75% circulating blast cells. Bone marrow aspirate reveals > 20% lymphoblasts staining positive for terminal deoxynucleotidyl transferase (TdT) and CD10. What is the diagnosis?

Answer and explanation

Correct answer: A. Acute Lymphoblastic Leukemia (ALL)

  1. Step 1: Acute Lymphoblastic Leukemia (ALL) is the most common childhood malignancy, accounting for ~75% of pediatric leukemias, peaking between 2 and 5 years of age.
  2. Step 2: Bone marrow infiltration results in pancytopenia (anemia, thrombocytopenia with petechiae, neutropenia) alongside bone pain and hepatosplenomegaly.
  3. Step 3: Diagnosis is confirmed by bone marrow aspirate demonstrating >= 20% lymphoblasts that express TdT and lineage-specific markers like CD10 (common ALL antigen / CALLA) and CD19.
Answer
Acute Lymphoblastic Leukemia (ALL).
Why not the others: 'Acute Myeloid Leukemia' blasts are characterized by Auer rods and positivity for myeloperoxidase (MPO); 'Infectious Mononucleosis' features atypical reactive lymphocytes without bone marrow suppression or severe thrombocytopenia; 'Aplastic Anemia' causes pancytopenia with a hypocellular fat-replaced marrow without circulating blast cells.

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