This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: ENT
[{'content': 'Mucocele most commonly occurs \nSelect and explain. \nA. Frontal sinus, B. Ethmoid sinus, C. Maxillary sinus, D. Sphenoid sinus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Mucoceles are dilated, mucous-containing sacs that are lined by mucous membranes and result from continuous secretions into an obstructed sinus. The frontal sinus is most commonly affected, whereas the sphenoid sinus is rarely involved. Sinus expansion with thinning and remodeling of the sinus walls occurs. When the frontal sinus is involved, as it expands downward and outward proptosis can occur. Ref : ENT textbook by Dhingra 6th edition Pgno : 198', 'role': 'assistant'}]