All of the following immunohistochemical markers are positive in the neoplatic cells of granulocytic sarcoma, except ?
**Core Concept:** Granulocytic sarcoma is a rare extramedullary tumor that occurs from the accumulation of immature myeloid cells. Immunohistochemical markers are used to identify the cell of origin in such tumors.
**Why the Correct Answer is Right:** Granulocytic sarcoma develops from immature myeloid cells, which typically express CD33, CD13, CD64, and CD34 markers. CD34 is a hematopoietic stem cell marker, and its absence helps to differentiate granulocytic sarcoma from its precursor, acute leukemia. CD34 is not expressed in the majority of acute myeloid leukemia (AML) cases, but it is present in hematopoietic stem cells and some cases of acute lymphoblastic leukemia (ALL).
**Why Each Wrong Option is Incorrect:**
A. CD34 is expressed in some cases of AML and ALL, making it incorrect as a single marker for granulocytic sarcoma.
B. CD13 is a marker for immature myeloid cells, making it incorrect because it is present in granulocytic sarcoma.
C. CD64 is a marker for immature myeloid cells, making it incorrect as a single marker for granulocytic sarcoma.
D. CD33 is a marker for immature myeloid cells, making it incorrect as a single marker for granulocytic sarcoma.
**Clinical Pearl / High-Yield Fact:** To diagnose granulocytic sarcoma, a panel of markers including CD33, CD13, CD64, and CD34 should be used. CD34 exclusion helps differentiate granulocytic sarcoma from acute leukemia. Granulocytic sarcoma is relatively common in AML patients with t(8;21) or t(15;17) chromosomal translocations, which results in fusion proteins (AML1-ETO and PML-RARα, respectively) and reduces CD34 expression.
**Correct Answer:** CD34 (Option A) is incorrect because it is expressed in some cases of AML and ALL, making it unreliable as a standalone marker for granulocytic sarcoma.