This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pelvis and Perineum 2, Subject name: Anatomy
A 29-year-old woman with a ruptured ectopic pregnancy is admitted to a hospital for culdocentesis. A long needle on the syringe is most efficiently inseed through which of the following structures? 
Select and explain. 
A. Anterior fornix of the vagina, B. Posterior fornix of the vagina, C. Anterior wall of the rectum, D. Posterior wall of the uterine body
B: Posterior fornix of the vagina
Answer: B: Posterior fornix of the vagina 
Explanation: A needle should be inseed through the posterior fornix, just below the posterior lip of cervix while the patient is in supine position, to aspirate abnormal fluid in the cul-de-sac of Douglas (rectouterine pouch). Rectouterine excavation is not most efficiently aspirated by puncture of other structures.