## **Core Concept**
Granulocytic sarcoma, also known as chloroma, is a rare extramedullary tumor composed of myeloid cells. It is often associated with acute myeloid leukemia (AML) or other myeloproliferative disorders. The diagnosis of granulocytic sarcoma relies on histopathological examination and immunohistochemical markers.

## **Why the Correct Answer is Right**
The correct answer, **C. CD117 (c-Kit)**, is a specific marker for granulocytic sarcoma because **CD117** is a receptor tyrosine kinase that is commonly expressed on the surface of myeloid cells. In the context of granulocytic sarcoma, **CD117** is used as a diagnostic marker to identify the myeloid lineage of the tumor cells. This is particularly useful in cases where the diagnosis is uncertain or when the tumor cells do not exhibit typical myeloid morphology.

## **Why Each Wrong Option is Incorrect**
- **Option A:** is incorrect because while **CD45** is a leukocyte common antigen and can be expressed on various types of leukocytes, it is not specific for granulocytic sarcoma and can be found on many other types of immune cells.
- **Option B:** is incorrect because **CD20** is primarily a marker for B cells and is not typically used to identify myeloid lineage cells such as those found in granulocytic sarcoma.
- **Option D:** is incorrect because **CD3** is a marker for T cells and is not relevant to the diagnosis of granulocytic sarcoma.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **granulocytic sarcoma** often presents in patients with **acute myeloid leukemia (AML)**, and the presence of this tumor can sometimes precede the diagnosis of AML. The use of **CD117 (c-Kit)** as a diagnostic marker can help in the early identification of myeloid lineage cells in these tumors.

## **Correct Answer:** . **CD117 (c-Kit)**