Review Question - QID 210501

A 23-year-old woman presents to her gynecologist for painful menses. Her menses are regular, occurring every 28 days and lasting approximately 4 days. Menarche was at age 12. Over the past 6 months, she has started to develop aching pelvic pain during the first 2 days of her menstrual period. Ibuprofen provides moderate relief of her symptoms. She denies nausea, vomiting, dyschezia, dyspareunia, irregular menses, or menses that are heavier than usual. Her past medical history includes an isolated episode of mild chlamydia treated 4 years ago as an outpatient without complications. She currently takes no medications. She works as a copywriter and does not smoke or drink. She has been in a monogamous relationship with her boyfriend for the past 3 years. They use condoms intermittently. Her temperature is 98.6°F (37°C), blood pressure is 111/69 mmHg, pulse is 92/min, and respirations are 18/min. Pelvic examination demonstrates a normal appearing vagina with no adnexal or cervical motion tenderness. The uterus is flexible and anteverted. Which of the following is the underlying cause of this patient's pain?