Final answer:
The nasal septum cannot appear within the maxillary sinus because it is the structure dividing the nasal cavity into two halves, whereas the maxillary sinus is an air-filled cavity within the maxillary bone, below the orbits.
Step-by-step explanation:
The one structure that cannot appear within the maxillary sinus is: a. Nasal septum.
The maxillary sinus is an air-filled cavity located within the maxillary bone, below the orbits. It has several boundaries: the floor of the nasal cavity which forms its base, the posterior wall which is toward the back of the sinus, and the anterior wall which is the front-facing surface. The nasal septum, on the other hand, is a structure that divides the nasal cavity into two halves and is not part of the maxillary sinus.
The maxillary sinuses are the largest of the paranasal sinuses and can commonly be involved in sinus infections. One of their notable characteristics is that because their connection to the nasal cavity is located high on their medial wall, they can be difficult to drain. The sinuses are lined with mucosa, which helps warm and humidify incoming air and also produce mucus that aids in protecting the respiratory system.