The mesial surface of which tooth is a particular challenge during endodontic access preparation, due to a developmental concavity that can complicate canal orifice location?

The maxillary first premolar's developmental concavity on its mesial surface (the same feature relevant to its periodontal plaque-trap risk) can also complicate endodontic access preparation, creating risk of perforation or missed canal orifices if the concavity isn't carefully accounted for during access cavity refinement.