This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nose and paranasal sinuses, Subject name: ENT
Sinus not present at bih is: 
Select and explain. 
A. Ethmoid, B. Maxillary, C. Sphenoid, D. Frontal
D: Frontal
Answer: D: Frontal 
Explanation: Frontal-Absent at bih-Adult size in20 years-Invades frontal bone(2-4yrs),size increases until teens. Ref.Scott Brown 7/e,Vol 2,p 1320.