Sinus least involved in sinusitis is:

Isolated involvement of sphenoid sinus is rare. It is often a pa of pansinusitis or is associated with infection of posterior ethmoidal sinus. The sphenoid sinus is rarely affected on its own --Turner 10th/ed p 48 The reason for sphenoid sinus to be least affected is that it opens high up in the sphero ethmoid recess which is not affected by most of the conditions of nose