Review Question - QID 213412

An 18-year-old girl comes to the emergency room with abdominal pain. She states that the pain started 6 hours ago, is 8/10, and localizes to the right lower abdomen. She recalls a similar episode last month. Her older brother was hospitalized 2 years ago for perforated appendicitis. Her mother has diverticulitis. The patient got her first menstrual period at age 14. Her periods occur regularly every 28 days. Her last menstrual period was 2 weeks ago. She is sexually active with multiple partners and uses condoms inconsistently. The patient’s temperature is 99°F (37.2°C), blood pressure is 110/74 mmHg, pulse is 89/min, and respirations are 14/min with an oxygen saturation of 98% on room air. On physical exam, the patient is in moderate distress. There is lower abdominal tenderness with guarding. A pelvic examination reveals clear mucous in the vaginal introitus, a mobile uterus, and tenderness of the right adnexa. Labs are obtained, as shown below:

Leukocyte count: 10,000/mm^3 with normal differential
Hemoglobin: 13.5 g/dL
Platelet count: 200,000/mm^3
Beta-human chorionic gonadotropin: Pending

A pelvic ultrasound demonstrates a small fluid collection in the cul-de-sac posterior to the uterus. Which of the following is the most likely diagnosis?