## Core Concept
The cardinal event of acute inflammation refers to the key process that characterizes the onset of acute inflammatory responses. Acute inflammation is a immediate, non-specific defense mechanism against tissue injury or infection. The cardinal signs of inflammation include redness, heat, swelling, pain, and sometimes loss of function.

## Why the Correct Answer is Right
The correct answer, **vasodilation**, is the cardinal event because it is the initial and pivotal change that occurs in the microcirculation during acute inflammation. Vasodilation leads to increased blood flow to the affected area, which in turn causes the classic signs of inflammation: redness (erythema) and heat (calor). This increased blood flow also brings more white blood cells to the site, which helps to fight off infection.

## Why Each Wrong Option is Incorrect
* **Option A:** While increased vascular permeability is a crucial event in acute inflammation that leads to edema (swelling), it is not considered the cardinal event. It occurs after vasodilation and allows for the exudation of fluid and proteins into the interstitial tissue.
* **Option B:** The migration of leukocytes (white blood cells) into the tissue is an essential process in acute inflammation but is not the cardinal event. This process occurs after vasodilation and increased vascular permeability have taken place.
* **Option D:** The release of chemical mediators is an important step in initiating the inflammatory response. However, it is not the cardinal event but rather one of the causes leading to vasodilation and other inflammatory changes.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the "five cardinal signs" of inflammation are redness, heat, swelling, pain, and loss of function. These are directly related to the underlying pathophysiological changes, with vasodilation being the initiating event that leads to the observable signs.

## Correct Answer: . Vasodilation