## **Core Concept**
Systemic mastocytosis is a disorder characterized by the accumulation of mast cells in one or more organs, particularly the skin, bone marrow, liver, spleen, and lymph nodes. The diagnosis often involves the use of specific markers to identify mast cells. One of the key markers used is CD117 (also known as c-KIT).

## **Why the Correct Answer is Right**
CD117 (c-KIT) is a receptor tyrosine kinase and a key marker for the diagnosis of systemic mastocytosis. It is used alongside other markers like CD25 and tryptase to confirm the presence of mast cells. CD117 is particularly important because mutations in the c-KIT gene can lead to the constitutive activation of the receptor, contributing to mast cell proliferation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** CD117 (c-KIT) is indeed a relevant marker.
- **Option B:** CD25 is also a relevant marker, especially when used in combination with CD117 for the diagnosis of systemic mastocytosis.
- **Option C:** Tryptase is a marker for mast cells and is relevant in the diagnosis.
- **Option D:** CD14 is not typically used as a diagnostic marker for systemic mastocytosis. CD14 is more commonly associated with myeloid cells and is used in different contexts, such as identifying monocytes.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the diagnostic criteria for systemic mastocytosis often involve a combination of major and minor criteria. The major criterion includes the presence of multifocal, dense infiltrates of mast cells (≥15 mast cells in aggregates) in bone marrow biopsy or other extracutaneous organ. Minor criteria include the presence of mast cells in peripheral blood, elevated serum total tryptase levels, and the detection of the D816V mutation in the KIT gene. The use of CD117 and other markers like CD25 helps in confirming the diagnosis.

## **Correct Answer:** D. CD14