**Question:** Fractalkine is also called

A. Chemokine (C-X3-C motif) ligand 1 (CX3CL1)
B. Chemokine (C-X3-C motif) ligand 2 (CX3CL2)
C. Chemokine (C-X3-C motif) receptor 1 (CX3CR1)
D. Chemokine (C-X3-C motif) receptor 2 (CX3CR2)

**Correct Answer:** A. Chemokine (C-X3-C motif) ligand 1 (CX3CL1)

**Core Concept:** Fractalkine is a chemokine, which is a type of signaling protein that plays a crucial role in immune system regulation and inflammation. It is involved in the recruitment and activation of immune cells, particularly monocytes and microglia, to sites of injury or infection.

**Why the Correct Answer is Right:** Fractalkine is also known as Chemokine (C-X3-C motif) ligand 1 (CX3CL1). It is a chemokine that acts as a chemoattractant for monocytes and microglia, thereby contributing to immune system regulation and inflammation. It is involved in the recruitment of immune cells to sites of injury or infection.

**Why Each Wrong Option is Incorrect:**

B. Chemokine (C-X3-C motif) ligand 2 (CX3CL2) - This is an incorrect choice as the correct term for fractalkine is CX3CL1, not CX3CL2.

C. Chemokine (C-X3-C motif) receptor 1 (CX3CR1) - This is also incorrect as CX3CR1 is the receptor for fractalkine, not the ligand (CX3CL1).

D. Chemokine (C-X3-C motif) receptor 2 (CX3CR2) - This is an incorrect choice as the correct receptor for fractalkine is CX3CR1, not CX3CR2.

**Clinical Pearl:** Fractalkine is crucial for immune cell recruitment and activation during inflammation and tissue repair processes. Understanding the correct nomenclature (CX3CL1) and its interaction with CX3CR1 is essential for understanding immune responses and inflammation-related medical conditions.