A 24-year-old woman comes to the emergency department due to lower abdominal cramping and vaginal bleeding. The patient started to have vaginal spotting a few days ago but has had slightly increased bleeding with painful lower abdominal cramping since this morning. She is sexually active, and her last menstrual period was 5 weeks ago. Blood pressure is 130/80 mm Hg and pulse is 100/min. Diffuse tenderness to palpation is present over the lower abdomen. On speculum examination, the cervix is closed and there is dark red blood coming from the cervical os. Bimanual examination reveals a small uterus and a 3-cm mass in the left adnexa. Quantitative serum β-hCG level is 1,200 mIU/mL (normal: <5). Which of the following is the most likely underlying cause of this patient's presentation?
1) Abnormal ovum fertilization by 2 sperm
2) Benign proliferation of the uterine myometrium
3) Endometrial tissue development within the ovary
4) Implantation of a fertilized embryo in the fallopian tube
5) Torsion of an ovarian mass containing ectoderm, endoderm, and mesoderm