A 19-year-old woman comes to the emergency room and reports that she fainted at work earlier in the day. She has mild vaginal bleeding. Her abdomen is diffusely tender and distended. In addition, she complains of shoulder and abdominal pain. Her temperature is 97.6degF, pulse rate is 120/min, and blood pressure is 96/50 mmHg. To confirm the diagnosis suggested by the available clinical data, the best diagnostic procedure is