A 26-year-old woman comes to the emergency department for pelvic pain and vaginal bleeding over the past 2 days. Today, the pain has increased, but she has had no nausea, vomiting, or orthostasis. The patient was treated for chlamydia cervicitis 3 years ago. She takes no medications and has no known drug allergies. On pelvic examination, the uterus is small and mobile and there is left adnexal tenderness. Pelvic ultrasound reveals a complex, 2-cm left adnexal mass with a gestational sac and yolk sac; there is no intrauterine pregnancy or free 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?

Question

A 26-year-old woman comes to the emergency department for pelvic pain and vaginal bleeding over the past 2 days.  Today, the pain has increased, but she has had no nausea, vomiting, or orthostasis.  The patient was treated for chlamydia cervicitis 3 years ago.  She takes no medications and has no known drug allergies.  On pelvic examination, the uterus is small and mobile and there is left adnexal tenderness.  Pelvic ultrasound reveals a complex, 2-cm left adnexal mass with a gestational sac and yolk sac; there is no intrauterine pregnancy or free 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?

Quizplus · Accepted Answer

11ec160b_3c47_eacc_9249_fb42e26497ed_MD0001_00 This patient with pelvic pain, vaginal bleeding, and a tender left adnexal mass containing a gestational sac and yolk sac has an ectopic pregnancy. Ectopic pregnancies occur due to pregnancy implantation in an extrauterine location, most commonly the fallopian tube. Risk factors include prior sexually transmitted infection (eg, Chlamydia trachomatis), prior abdominal surgery, and prior ectopic pregnancy. Medical treatment for an early, unruptured ectopic pregnancy is with methotrexate, a folic acid antagonist. Methotrexate works by directly competing with folic acid to bind dihydrofolate reductase, thereby inhibiting the formation of tetrahydrofolate. This decrease in tetrahydrofolate, an integral precursor to purine and pyrimidine synthesis, inhibits DNA synthesis and cell reproduction. The result is preferential destruction of actively proliferating cells, such as fetal cells and trophoblasts. (Choice A) During pregnancy, the uterus increases its number of progesterone receptors so that circulating progesterone can stimulate the endometrial and myometrial growth required for pregnancy implantation and development. Mifepristone, which acts as a progesterone antagonist when progesterone levels are high, is used for medical abortion because it causes necrosis of the uterine decidua and stimulates uterine contractions. (Choice B) Progestins (eg, levonorgestrel) thin the endometrial lining and decrease LH secretion, thereby inhibiting ovulation and preventing pregnancy. They are not used in ectopic pregnancy management. (Choice C) Misoprostol is a synthetic prostaglandin E1 agonist that acts on the uterus to induce contractions; it can be used for labor induction or medical abortion. (Choice E) Copper-containing intrauterine devices produce local inflammation to impair intrauterine implantation. They are not used to treat ectopic pregnancies, which have implanted outside the uterus. Educational objective: Ectopic pregnancy occurs due to pregnancy implantation in an extrauterine location (eg, fallopian tube). Medical treatment is with methotrexate, which inhibits DNA synthesis and cell reproduction, primarily in actively proliferating cells such as fetal cells and trophoblasts. __________ References: Medical management of ectopic pregnancy: a comparison of regimens. (http://www.ncbi.nlm.nih.gov/pubmed/22510626)

A 26-Year-Old Woman Comes to the Emergency Department for Pelvic