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
What is a placental cotyledon? 
Select and explain. 
A. All branches from one stem villi, B. Area supplied by one spiral aery, C. Quaer of placenta, D. Area drained by one terminal villi
A: All branches from one stem villi
Answer: A: All branches from one stem villi 
Explanation: 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.