The sphenoid sinus.
D003937:Diagnosis, Differential; D006801:Humans; D010255:Paranasal Sinus Neoplasms / Q000175:diagnosis / Q000601:surgery*; D012852:Sinusitis / Q000175:diagnosis* / Q000601:surgery* / Q000628:therapy*; D013101:Sphenoid Sinus / Q000033:anatomy & histology*; D007507:Therapeutic Irrigation