## **Core Concept**
Chemotaxis refers to the movement of cells, particularly white blood cells, toward or away from a chemical stimulus. In the context of inflammation and immune response, chemotaxis is crucial for the recruitment of leukocytes to sites of infection or injury. The process is primarily mediated by a family of chemotactic factors.

## **Why the Correct Answer is Right**
The correct answer, **Leukotriene B4 (LTB4)** and more prominently **C5a** and **C3a** (but most specifically **C5a**), are potent chemotactic agents. However, among the given options, if we consider the broader and most direct mediator of chemotaxis, **C5a** stands out. C5a is a fragment generated from the complement component C5 during the activation of the complement system. It acts through specific receptors on the surface of leukocytes, inducing a strong chemotactic response, particularly for neutrophils and monocytes. This response is critical for the early stages of inflammation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain **Leukotrienes**, such as LTB4, are chemotactic, they are not considered the most important mediators compared to the complement factors.
- **Option B:** This option might seem plausible but without specific details; it's hard to assess directly. However, given the context, it's likely less directly involved in chemotaxis compared to C5a.
- **Option D:** Similarly, without specifics, it's challenging to evaluate, but generally, **C3a** has more anaphylatoxin properties than potent chemotactic effects compared to C5a.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **C5a** not only induces chemotaxis but also activates leukocytes, enhancing their adhesion to endothelial cells and increasing their expression of adhesion molecules. This dual role makes C5a a pivotal mediator in the inflammatory response.

## **Correct Answer:** C. C5a.