**Core Concept**
Inflammation is a complex biological response to tissue damage or infection, characterized by five cardinal signs: rubor (redness), calor (heat), tumor (swelling), dolor (pain), and functio laesa (loss of function). Rubor is a result of increased blood flow to the affected area due to vasodilation.

**Why the Correct Answer is Right**
The correct answer is due to the release of various chemical mediators, such as histamine, bradykinin, and prostaglandins, which stimulate the smooth muscle cells in blood vessels to relax, leading to vasodilation. This increased blood flow causes the affected area to become red and warm, resulting in rubor. The increased permeability of blood vessels also contributes to the exudation of fluid and proteins into the interstitial space, further contributing to the redness.

**Why Each Wrong Option is Incorrect**
**Option A:** Vasospasm would actually decrease blood flow to the affected area, leading to pallor rather than rubor.
**Option B:** Edema is a result of increased fluid and protein exudation into the interstitial space, which contributes to swelling (tumor) but not directly to rubor.
**Option C:** Pain (dolor) is a separate cardinal sign of inflammation, caused by the activation of nociceptors and the release of chemical mediators such as bradykinin.

**Clinical Pearl / High-Yield Fact**
Remember that the sequence of the cardinal signs of inflammation is often remembered using the mnemonic "REDBLACK" - Rubor, Exudation, Dolor, Blancheur (pallor), Loss of function, Calor, and Anasarca (swelling).

**Correct Answer: A. Vasodilation.