A 31-year-old woman comes to the emergency depament because of abdominal pain and vaginal spotting. She states that the pain began 2 days ago and has been worsening since. The spotting occurred this morning. Her last menstrual period was 6 weeks ago. She has no medical problems. She takes no medications and has no known drug allergies. Her temperature is 37 C (98.6 F), blood pressure is 90/50 mm Hg, pulse is 110/minute, and respirations are 14/minute. Abdominal examination reveals significant lower abdominal tenderness. Speculum examination shows scant blood in the vagina with a closed cervical os. Bimanual examination demonstrates significant left adnexal tenderness. Urine hCG is positive. Serum hCG is 5,000 mIU/mL. Pelvic ultrasound shows a normal uterus with a left adnexal mass surrounded by free fluid. In MOST cases, the disease process of this patient affects which of the following anatomic structures?

**Core Concept**
The clinical presentation suggests a diagnosis of **ectopic pregnancy**, which is a life-threatening condition where a fertilized egg implants outside the **uterus**, commonly in the **fallopian tube**. This condition is often associated with abdominal pain, vaginal bleeding, and positive **human chorionic gonadotropin (hCG)** levels.

**Why the Correct Answer is Right**
In most cases of ectopic pregnancy, the disease process affects the **fallopian tube**, leading to tubal rupture or implantation of the embryo. The **fallopian tube** is the most common site for ectopic pregnancies, and the presentation of abdominal pain, vaginal spotting, and adnexal tenderness is consistent with this diagnosis. The positive **urine hCG** and **serum hCG** levels, along with the **pelvic ultrasound** findings, support this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, although the **ovary** can be involved in ectopic pregnancies, it is not the most common site.
**Option B:** Incorrect because the **uterus** is not typically the site of implantation in ectopic pregnancies.
**Option D:** Incorrect because, while the **bowel** can be affected by the disease process due to **hemorrhage** or **infection**, it is not the primary site of implantation.

**Clinical Pearl / High-Yield Fact**
Ectopic pregnancy is a medical emergency that requires prompt diagnosis and treatment to prevent **maternal mortality**. A key clinical pearl is to consider ectopic pregnancy in any woman of childbearing age presenting with abdominal pain and vaginal bleeding, especially with a positive **hCG** level.

**Correct Answer:** C. Fallopian tube.

✓ Correct Answer: B. Fallopian tube