Shelly, a 23-year-old female, presents to the emergency room with severe abdominal pain. She is currently 7 weeks pregnant with her second child and did not experience any problems during her first pregnancy. She has not noticed having any vaginal bleeding in the preceding days. Upon examination, her physician notes that she is slightly hypotensive with a normal pulse rate and her abdomen is distended and tender on the right side. A pelvic examination reveals that her cervix is closed and her uterus is consistent with expectations for a person who is 7 weeks pregnant. A blood test reveals that she is positive for beta-human chorionic gonadotropin, but that her hemoglobin levels have dropped since her prenatal appointment 3 days ago. Her physician orders an ultrasound, which reveals a viable singleton pregnancy in the uterus and a large amount of free fluid within the abdomen. A right adnexal mass is noted. Shelly is rushed to the operating room. what is her diagnosis?