Among permanent teeth, which one presents the greatest challenge in differentiating the mesial surface from the distal surface?

Explanation

The mandibular central incisor is the permanent tooth where distinguishing between the mesial and distal surfaces is most difficult due to its symmetrical shape. Other teeth, like the maxillary central incisor, mandibular lateral incisor, and mandibular second premolar, have more distinct mesial and distal features.