A 42-year-old woman attends for a routine check-up. The lower left second premolar has a large old composite restoration. There is a painless yellow papule on the buccal attached gingiva near the root apex that discharges pus on pressure. The tooth does not respond to cold or to the electric pulp tester, is not tender to percussion, and the radiograph shows a 5 mm periapical radiolucency. Periodontal probing depths are 2 to 3 mm. Which diagnosis is most appropriate?