A 9-year-old boy has an anterior mediastinal mass, anemia, and circulating blasts that are negative for myeloperoxidase and Sudan black B. Flow cytometry shows cytoplasmic CD3 positivity and strong nuclear positivity for which enzyme would confirm the immature lymphoid nature of these blasts?

• A Terminal deoxynucleotidyl transferase (TdT) ✓
• B Lysozyme
• C Tartrate-resistant acid phosphatase
• D Alpha-naphthyl butyrate esterase

Explanation

TdT is a specialized DNA polymerase expressed only by immature lymphoid cells, both B and T lineage, and absent from mature lymphocytes and myeloid cells. Its nuclear positivity supports a diagnosis of lymphoblastic leukemia or lymphoma. Cytoplasmic CD3 confirms T lineage, and the mediastinal mass fits T-lymphoblastic lymphoma arising from thymic cortical thymocytes. Lysozyme and nonspecific esterase mark monocytes, and TRAP marks hairy cell leukemia.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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