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A 37-year-old woman, gravida 7 para 2 aborta 4, comes to the hospital with labor pains. The patient received care at another clinic, but her medical records are unavailable. She delivered her first child vaginally at age 16 after an uncomplicated, full-term pregnancy. The patient subsequently had 4 elective terminations followed by a cesarean delivery at term for breech presentation. On physical examination, her cervix is 6 cm dilated and 90% effaced with the fetal head at +1 station. She is hospitalized, dilates quickly to 10 cm, and vaginally delivers a healthy 4540-g (10-lb) boy. The umbilical cord avulses from the placenta, necessitating a manual extraction. The placenta is extracted in pieces. During placental removal, the patient develops profuse vaginal bleeding. Her uterus is firm, and the bleeding is unresponsive to uterine massage and uterotonic medications. Which of the following is the most likely cause of her bleeding?