All are of adult size at bih except: (A) Mastoid antrum, (B) Ear Ossicles, (C) Tympanic cavity, (D) Maxillary antrum
Answer is D. 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.