Q. A 30-year-old woman, gravida 3 para 2, at 30 weeks gestation, presents to the emergency department with painless, bright red vaginal bleeding that started suddenly earlier in the day. She reports no abdominal pain, trauma, or contractions. Her previous pregnancies were uncomplicated and resulted in normal vaginal deliveries. Her prenatal course for this pregnancy has been unremarkable. Her vital signs are stable with a blood pressure of 120/80 mmHg and a heart rate of 86 beats per minute. On examination, the abdomen is soft, and non-tender, and the uterus is appropriately sized for gestational age with a fetal heart rate of 145 beats per minute. Pelvic examination shows bright red vaginal bleeding. What is the most likely source of bleeding in this patient?