Which tooth has a MESIAL root concavity visible on the mesial surface?

The maxillary first premolar is classic for having a prominent mesial root concavity (developmental depression) on the mesial surface of the root — sometimes called the 'canine fossa of the premolar.' This groove can extend from the cervical area toward the apex and makes root planing difficult and creates a risk of root perforation during preparation. It is a critical anatomical landmark for endodontic and periodontal treatment.