Patient: 31-year-old woman. Chief complaint: occasional bad taste near her upper left front teeth. History: healthy nonsmoker with good oral hygiene. The periodontal chart of the maxillary anterior sextant is shown in the image. Radiograph: a narrow radiolucent area along the root of #7 and a faint vertical line over the root. #7 has no caries or restorations. What is the most likely cause of the finding on #7?