**Core Concept**
Chlorhexidine is a broad-spectrum antimicrobial agent commonly used in oral care products, including mouthwashes and toothpaste, to prevent plaque formation and gingivitis. Its mechanism of action involves disrupting the bacterial cell membrane, ultimately leading to cell death.

**Why the Correct Answer is Right**
The correct answer is related to the staining properties of chlorhexidine on tooth surfaces. When chlorhexidine comes into contact with tooth enamel, it can precipitate out of solution and adhere to the tooth surface, causing a characteristic brown or black discoloration. This is due to the high affinity of chlorhexidine for tooth enamel and its resistance to being removed by saliva or oral hygiene practices.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because chlorhexidine is not typically associated with tooth decay or caries.
**Option B:** Incorrect because chlorhexidine is not a known allergen, and allergic reactions to it are rare.
**Option C:** Incorrect because chlorhexidine is not typically used as a desensitizing agent for sensitive teeth.

**Clinical Pearl / High-Yield Fact**
To minimize the risk of chlorhexidine staining, patients should be advised to spit out the mouthwash or toothpaste after use, rather than swallowing it, and to rinse their mouth thoroughly with water.

**Correct Answer: D. Chlorhexidine staining of tooth surfaces.**