A 26-Year-Old Woman Is Brought to the Emergency Department Because

A 26-year-old woman is brought to the emergency department because of a 1-day history of lower abdominal pain and vaginal bleeding. Her last menstrual period was 8 weeks ago. A urine pregnancy test is positive. A pelvic ultrasound shows a normal appearing uterus with an empty intrauterine cavity and a minimal amount of free pelvic fluid. The patient is counseled on recommended medical treatment.  Which of the following is the mechanism of action of the agent of choice for this patient?

A) Blocks progesterone receptors, resulting in uterine contractions
B) Decreases LH secretion, causing endometrial thinning
C) Induces prostaglandin-mediated uterine contractions
D) Inhibits DNA synthesis to destroy actively proliferating fetal cells
E) Produces local inflammation to impair implantation