Annexin V is a marker of
**Core Concept**
Annexin V is a protein that binds to phosphatidylserine (PS), a phospholipid component of cell membranes. This binding is specific and reversible, making Annexin V a useful marker for detecting cell membrane changes associated with apoptosis (programmed cell death).
**Why the Correct Answer is Right**
Annexin V binds to exposed PS on the outer leaflet of the cell membrane, which is a characteristic feature of apoptotic cells. This exposure is due to the activation of caspases, which cleave and inactivate flippase, a protein responsible for maintaining PS asymmetry in the cell membrane. As a result, PS is translocated to the outer leaflet, where it can be detected by Annexin V.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the context of Annexin V as a marker.
**Option B:** While apoptosis is indeed related to cell membrane changes, Annexin V is not a marker for necrosis.
**Option C:** This option is not related to the specific binding properties of Annexin V.
**Clinical Pearl / High-Yield Fact**
Annexin V staining is often used in conjunction with propidium iodide (PI) staining to distinguish between apoptotic and necrotic cells. Apoptotic cells will be Annexin V positive and PI negative, while necrotic cells will be both Annexin V and PI positive.
**Correct Answer: A. Apoptosis.**