A 35-year-old man has a deeply carious lower left first molar. Cold testing causes sharp pain that lingers for about 20 seconds, and the tooth is slightly tender to biting. The periapical radiograph shows a diffuse, uniform radiopacity around the apex of the mesial root; the periodontal ligament space is continuous, the root outline is intact and there is no radiolucent rim. What is the most appropriate diagnosis and management?