A bone marrow biopsy in a patient with ALL shows positive staining for TdT (terminal deoxynucleotidyl transferase). What does TdT positivity indicate?
- A Myeloid lineage commitment
- B Immature lymphoid precursor (pre-B or pre-T) phenotype ✓
- C Plasma cell differentiation
- D Natural killer cell origin
Correct answer: B. Immature lymphoid precursor (pre-B or pre-T) phenotype
Explanation
TdT is a DNA polymerase expressed only in immature (precursor) B and T lymphoblasts; it adds random nucleotides at V(D)J recombination junctions to generate antibody/TCR diversity. Its expression confirms lymphoid precursor origin and is a hallmark of lymphoblastic leukemia/lymphoma. Myeloid cells, plasma cells, and NK cells do not express TdT.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.