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: Anatomy
All are of adult size at bih except: 
Select and explain. 
A. Mastoid antrum, B. Ear Ossicles, C. Tyrnpanic cavity, D. Maxillary antrum
D: Maxillary antrum
Answer: D: Maxillary antrum 
Explanation: Maxillary antrum "The tympanic cavity and mastoid antrum , auditory ossicles and structures of the internal ear are all almost fully developed at bih and subsequently alter little." -Gray's Anatomy 40/e p623Maxillary antrum reaches adult size by 15 yrs of age. All paranasal sinuses except maxillary (and sometimes ethmoidal) are rudimentary or absent at bih. They enlarge appreciably during the eruption of the permanent teeth and after pubey, events that significantly alter the size and shape of the face.Some students were of the opinion that in the present question 'orbit' was the option instead of maxillary antrum. In that case the answer would be orbit. "Orbit reaches its full adult size by 15 to 16 yrs of age." Also Know:Though the mastoid antrum is well developed at bih, the mastoid air cells are not developed and are merely minute antral diveicula at this stage. The mastoid process develops in the 2"d yr and is invaded by air cells in the 6ill yr.(Ref: Grays 40/e p417)