A radiograph shows a lower premolar with an enlarged canal. Which statement is true?

An enlarged canal in a lower premolar on a radiograph is indicative of internal resorption. This condition occurs when the tooth's internal structure is compromised, typically due to factors such as chronic inflammation or trauma. In internal resorption, the dentin and potentially the pulp become eroded, resulting in an increase in the size of the canal within the tooth.

Normal anatomy would not present with an enlarged canal, as this is typically characteristic of a healthy tooth where the canal size remains consistent. External resorption, on the other hand, involves the loss of tooth structure from the outside, usually around the root area, and would generally not lead to an enlargement of the canal itself. Caries and periapical pathosis could also cause changes in the overall structure of the tooth, but they would not specifically result in an enlarged canal without additional factors like pulp involvement.

In summary, the enlargement of the canal suggests a pathological process affecting the internal tissues of the tooth, reaffirming that internal resorption is the correct interpretation of the radiographic finding.