## Core Concept
Walking bleach, also known as internal bleaching or non-vital bleaching, is a dental procedure used to lighten a tooth that has been discolored due to a root canal or other dental work. This technique involves placing a bleaching agent inside the tooth through a small opening.

## Why the Correct Answer is Right
The correct answer, ., is not an indication for walking bleach because walking bleach is specifically used for non-vital teeth that have been discolored due to root canal treatment or trauma. If a tooth is vital (., option D), walking bleach is not applicable as the procedure requires access to the pulp chamber which is typically only done in non-vital teeth.

## Why Each Wrong Option is Incorrect
- **Option A:** . is a valid indication for walking bleach. Discoloration due to endodontic treatment is a common reason for this procedure.
- **Option B:** . is also a valid indication. Trauma can cause discoloration in teeth, and walking bleach can be used to improve the tooth's appearance.
- **Option C:** . can be an indication as well. Although less common, certain materials or procedures related to dental work can cause tooth discoloration that walking bleach can address.

## Clinical Pearl / High-Yield Fact
A key point to remember is that walking bleach is only suitable for non-vital teeth. A critical clinical consideration is to ensure the tooth is properly sealed after the procedure to prevent leakage of the bleaching agent and potential damage to the tooth or surrounding tissues.

## Correct Answer: D. Vital tooth.