This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nose and paranasal sinuses, Subject name: ENT
Sinusitis which is least involved is? 
Select and explain. 
A. Maxillary, B. Ethmoid, C. Frontal, D. Sphenoid
D: Sphenoid
Answer: D: Sphenoid 
Explanation: Sinusitis is least involved in the sphenoid sinus. Isolated involvement of this sinus is rare. It's often pa of pansinusitis or is associated with infection of posterior ethmoid sinuses. (Reference: Textbook of PL Dhingra and Shruti Dhingra; 7th edition, pg no. 216)