Flow cytometry on a bone marrow aspirate from a 6-year-old boy with acute leukemia shows blasts positive for terminal deoxynucleotidyl transferase (TdT), cytoplasmic CD3, and CD7, with surface CD3 absent. This profile establishes lineage as:
- A Natural killer cell neoplasm, since surface CD3 is absent
- B B lymphoblastic, since TdT marks all lymphoid precursors
- C T lymphoblastic, since cytoplasmic CD3 is the lineage-defining marker ✓
- D Indeterminate, requiring cytogenetics before classification
Correct answer: C. T lymphoblastic, since cytoplasmic CD3 is the lineage-defining marker
Explanation
Cytoplasmic CD3 expression is the most specific lineage-defining marker for T-cell origin and is present before surface CD3 appears during thymic maturation. TdT merely indicates immaturity and is shared by both B- and T-lymphoblastic leukemias, so option B fails. Absence of surface CD3 reflects an immature stage of T-cell differentiation rather than NK lineage. Cytogenetics refine risk stratification but do not determine lineage here.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.