This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: ENT
Sinus not present at bih is/are: September 2010 
Select and explain. 
A. Maxillary, B. Ethmoidal, C. Frontal, D. All of the above
C: Frontal
Answer: C: Frontal 
Explanation: Ans. C: Frontal The paranasal sinuses are hollow, air-filled cavities that are lined by a mucous membrane. The ethmoid and maxillary sinuses are present at bih. The frontal sinus develops about the second year of life and the sphenoid about the third year. Sometimes, the frontal sinuses do not develop.