This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: ENT
All of the following structures open into the middle meatus, EXCEPT: 
Select and explain. 
A. Ant. ethmoidal sinus, B. Post. ethmoidal sinus, C. Frontonasal duct, D. Maxillary sinus
B: Post. ethmoidal sinus
Answer: B: Post. ethmoidal sinus 
Explanation: Posterior ethmoid sinus opens into superior meatus. Anterior ethmoid cells are divided into frontal recess cells, which open into the frontal recess of the middle meatus, infundibular cells which open into the ethmoid infundibulum and and bullae or middle ethmoid cells which open into the middle meatus. Maxillary sinus drain into the middle meatus of nasal cavity. The frontal sinus opens into the infundibulum of middle meatus through frontonasal duct. Sphenoid sinus opens into the sphenoethmoidal recess above and behind the superior nasal concha.