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
Which of the following structure is NOT present at bih? 
Select and explain. 
A. Ethmoid sinus, B. Mastoid tip, C. Incus, D. Malleus
B: Mastoid tip
Answer: B: Mastoid tip 
Explanation: Mastoid tip is absent at the time of bih but mastoid antrum is present. Postnatally, mastoid air cells develop leading to formation of mastoid process (by about 2 years ago). At bih, only the maxillary and ethmoidal sinuses are present and are large enough to be clinically significant. Malleus and incus are derived from Meckel's cailage of the first branchial (pharyngeal) arch before bih. .