Which surface of the maxillary first premolar is most difficult to clean and most prone to plaque accumulation, due to a developmental anatomic feature?

The maxillary first premolar classically has a developmental concavity/groove on its mesial surface, creating a localized plaque trap that makes this surface notably more difficult to clean than the other surfaces — a recognized local anatomic risk factor for periodontal breakdown in that specific area if oral hygiene is inadequate.