## **Core Concept**
Diapedesis refers to the process by which cells, particularly **leukocytes** (white blood cells), move through the intact walls of blood vessels into the tissues. This is a critical aspect of the inflammatory response, allowing immune cells to reach sites of infection or injury.

## **Why the Correct Answer is Right**
The correct answer, **C. migration of leukocytes through the intact vessel wall**, accurately describes diapedesis. This process involves the adhesion of leukocytes to the endothelial lining of blood vessels, followed by their migration through the endothelial layer and into the surrounding tissue. This is facilitated by various adhesion molecules and chemokines that guide the leukocytes to the site of inflammation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Passage of fluid through vessel wall - This describes **transudation**, not diapedesis. Transudation is the process by which fluid moves through the vessel wall due to hydrostatic pressure differences, leading to edema.
- **Option B:** Increased permeability of vessel wall - While increased permeability of the vessel wall can be a part of the inflammatory response and can facilitate diapedesis, it is not the definition of diapedesis itself. Increased permeability allows both fluid and cells to move through the vessel wall.
- **Option D:** Leakage of fluid through the damaged vessel wall - This describes **exudation**, which occurs due to damage to the vessel wall and results in the leakage of fluid, proteins, and sometimes blood cells into the tissues.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl related to diapedesis is that it is an essential component of the body's defense mechanism, enabling leukocytes to reach and combat infections or injury in tissues. Disorders affecting diapedesis can lead to impaired immune responses, making patients more susceptible to infections.

## **Correct Answer:** C. migration of leukocytes through the intact vessel wall