This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anatomy of the female genital tract, Subject name: Gynaecology & Obstetrics
A 26-year-old G2P2 woman underwent a normal vaginal delivery 10 days ago. She comes into the doctor's clinic complaining of a large amount of bright red bleeding beginning since 5 PM the previous day. Which of the following is the most likely diagnosis 
Select and explain. 
A. Uterine atony, B. Subinvolution of the uterus, C. Vaginal laceration, D. Cervical laceration
B: Subinvolution of the uterus
Answer: B: Subinvolution of the uterus 
Explanation: The most common cause of late postpaum hemorrhage is subinvolution of the uterus, in which the placental implantation site does not decrease in size as expected; thus, when the eschar overlying the placental site falls off (7- 10 days after delivery), there is more bleeding than expected. The treatment is uterotonic agents such as ergot alkaloids or misoprostol. The bleeding almost always decreases quickly, such as within 12 hours.