Professional and Linguistic Assessments Board (PLAB) Practice Exam

A patient showing a positive TdT stain is most likely diagnosed with which condition?

1. Acute Myeloid Leukemia

2. Aplastic Anemia

3. Acute Lymphoblastic Leukemia (ALL)

4. Chronic Lymphocytic Leukemia

The correct answer is: Acute Lymphoblastic Leukemia (ALL)

A positive TdT (terminal deoxynucleotidyl transferase) stain is indicative of acute lymphoblastic leukemia (ALL). TdT is an enzyme that is typically expressed in immature lymphoid progenitor cells, particularly in lymphoblastic cells seen in ALL. The presence of TdT serves as a marker for these malignant lymphoid cells and helps distinguish them from myeloid cells or mature lymphocytes, which do not express this enzyme. In the context of hematologic malignancies, the positive TdT stain is a cornerstone in the identification of acute lymphoblastic leukemia, making it critical in the diagnostic process. The unique expression of TdT in this condition signifies the presence of precursor B or T lymphocytes characteristic of ALL, which helps guide clinical management and treatment strategies. Other conditions listed do not typically show positive TdT staining. For example, acute myeloid leukemia (AML) generally arises from myeloid lineage cells that do not express TdT. Aplastic anemia primarily affects hematopoietic stem cells but does not involve malignant transformation associated with TdT. Lastly, chronic lymphocytic leukemia (CLL) is characterized by mature B-lymphocytes that do not demonstrate TdT positivity either.