## **Core Concept**
Rubor, or redness, is one of the cardinal signs of inflammation. It occurs due to increased blood flow to the affected area. This phenomenon is primarily mediated by the dilation of blood vessels.

## **Why the Correct Answer is Right**
The correct answer, **D. Vasodilation**, is right because rubor in inflammation is primarily caused by the dilation of arterioles, which leads to increased blood flow to the affected area. This increased blood flow brings more white blood cells to the site of injury or infection, which is a crucial part of the inflammatory response. The dilation of blood vessels is mediated by various chemical mediators released during the inflammatory process, such as histamine, bradykinin, and prostaglandins.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while certain cells and chemicals are involved in the inflammatory process, the direct cause of rubor is not explicitly listed here.
- **Option B:** This option is incorrect because increased blood viscosity would actually decrease blood flow, not increase it, and is not directly related to the cause of rubor.
- **Option C:** This option is incorrect because while congestion (or increased blood flow) is related to rubor, it is not the cause; rather, it is a result of vasodilation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the four cardinal signs of inflammation are rubor (redness), calor (heat), tumor (swelling), and dolor (pain). Rubor and calor are primarily due to **vasodilation** and increased blood flow to the affected area.

## **Correct Answer:** D. Vasodilation.