Review Question - QID 210508

A 29-year-old G2P1 at 35 weeks gestation presents to the obstetric emergency room with vaginal bleeding and severe lower back pain. She reports the acute onset of these symptoms 1 hour ago while she was outside playing with her 4-year-old son. Her prior birthing history is notable for an emergency cesarean section during her first pregnancy. She received appropriate prenatal care during both pregnancies. She has a history of myomectomy for uterine fibroids. Her past medical history is notable for diabetes mellitus. She takes metformin. Her temperature is 99.0°F (37.2°C), blood pressure is 104/68 mmHg, pulse is 120/min, and respirations are 20/min. On physical examination, the patient is in moderate distress. Large blood clots are removed from the vaginal vault. Contractions are occurring every 2 minutes. Delayed decelerations are noted on fetal heart monitoring. Which of the following is the most likely cause of this patient's symptoms?