This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Congenital Lesions and Stridor, Subject name: ENT
[{'content': 'The marked structure on the X-ray is: \nSelect and explain. \nA. Nasopharynx, B. Ethmoidal air cells, C. Sphenoid sinus, D. Pituitary gland', 'role': 'user'}, {'content': 'Answer: C \nExplanation: (c) Sphenoid sinus(Ref. Scott Brown, 6th ed., 4/3/2)Shown below are X-ray Waters view (occipitomental) with open mouth and X-ray Nose & PNS- lateral view. Note the highest sinus in the lateral view is the frontal sinus.', 'role': 'assistant'}]