**Core Concept**
Internal resorption is a type of tooth resorption that occurs within the dental pulp, leading to the destruction of dentin and potential exposure of the pulp cavity. This process is characterized by the activation of odontoclasts, which are specialized cells responsible for the breakdown of dentin.

**Why the Correct Answer is Right**
Internal resorption is often radiographically visible as a well-defined radiolucent area within the tooth, which may be surrounded by a radiopaque border. This is due to the activation of odontoclasts, which break down the dentin and create a cavity within the tooth. The radiolucent area may be accompanied by the formation of a radiopaque layer, which represents the remaining dentin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because external resorption typically presents as a radiolucent area surrounding the tooth, rather than within the tooth itself.
**Option B:** This option is incorrect because caries typically presents as a mixture of radiolucent and radiopaque areas, with a well-defined border between healthy and affected tooth structure.
**Option C:** This option is incorrect because pulp calcification typically presents as a radiopaque area within the tooth, rather than a radiolucent area.

**Clinical Pearl / High-Yield Fact**
Internal resorption is often associated with trauma to the tooth, and may be a sign of an underlying infection or inflammation within the pulp.

**Correct Answer: C. A radiolucent area within the tooth, surrounded by a radiopaque border.**