This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Anatomy
[{'content': 'What is a placental cotyledon? \nSelect and explain. \nA. All branches from one stem villi, B. Area supplied by one spiral aery, C. Quaer of placenta, D. Area drained by one terminal villi', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Each of the truncal or main stem villi and their ramifications constitutes a placental lobule or cotyledon. Each lobule is supplied with a single truncal branch of chronic aery. And each lobule has a single vein so that lobules constitute functional units of placental architecture. Ref: Williams 24e pg 94.', 'role': 'assistant'}]