Which of the following injections is needed to anesthetize the maxillary posterior teeth?

The correct choice for anesthetizing the maxillary posterior teeth is the PSA nerve block. This block specifically targets the posterior superior alveolar nerve, which provides sensation to the maxillary molars, typically including the first, second, and sometimes the third molars in the upper jaw. By delivering anesthesia through this block, you effectively numb the areas around these teeth, allowing for pain-free dental procedures in that region.

In contrast, the IA (Inferior Alveolar) nerve block primarily targets the mandibular teeth in the lower jaw, making it ineffective for maxillary posterior teeth. The MSA (Middle Superior Alveolar) nerve block is designed to anesthetize the maxillary premolars and the mesial root of the maxillary first molar, but not the posterior teeth as a whole. The GP (Greater Palatine) nerve block mostly anesthetizes the hard palate and is less effective for specific tooth pain, thus not suited for numbing the maxillary posterior teeth directly. Therefore, the PSA nerve block is the optimal choice for ensuring effective anesthesia for the maxillary posterior region.