A 32-year-old multiparous woman in active labor (cervical dilation 6 cm, contractions every 2–3 minutes) reports sudden onset of severe, tearing abdominal pain and vaginal bleeding. Fetal heart rate drops from 140 to 100 bpm with late decelerations. On palpation, the uterus is tender and rigid. Which investigation is most appropriate to confirm the suspected diagnosis and guide emergency management?