<prompt> Which of the following structure is NOT present at bih?Ethmoid sinusMastoid tipIncusMalleus  <response>: 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 Meckels cailage of the first branchial pharyngeal arch before bih. . 