This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
[{'content': 'Which of the following is not present at bih? \nSelect and explain. \nA. Mastoid tip, B. Mastoid antrum, C. Ethmoidal labrynth, D. Endolymphatic sinus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The mastoid process is absent or rudimentary in neonatal skull. It forms postnatally as sternocleidomastoid develops and pulls on the bone Web search', 'role': 'assistant'}]