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
The marked structure on the X-ray is: 
Select and explain. 
A. Nasopharynx, B. Ethmoidal air cells, C. Sphenoid sinus, D. Pituitary gland
C: Sphenoid sinus
Answer: C: Sphenoid sinus 
Explanation: (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.