Which structure may NOT be anesthetized by the PSA nerve block?

The correct response highlights that the mesiobuccal (MB) root of the maxillary first molar may not be effectively anesthetized by the posterior superior alveolar (PSA) nerve block.

The PSA nerve primarily innervates the maxillary molars, specifically the maxillary second and third molars, as well as portions of the buccal gingiva in that area. However, the MB root of the maxillary first molar is typically innervated by the middle superior alveolar (MSA) nerve in individuals who possess this anatomical variation. The MSA nerve, when present, provides sensation to the MB root of the first molar, making it necessary to ensure that this area is sufficiently anesthetized by alternatives such as an MSA block or an infiltration if the MSA nerve is absent.

Therefore, while the PSA nerve block is effective for the maxillary second and third molars, reliance on it for complete anesthesia of the MB root of the maxillary first molar is not sufficient, underscoring the need to assess the specific innervation of the target tooth.