You are answering a USMLE-style medical question. Question: A 15-year-old girl comes to the physician because of episodic pelvic pain radiating to her back and thighs for 4 months. The pain occurs a few hours before her menstrual period and lasts for 2 days. She has been taking ibuprofen, which has provided some relief. Menses have occurred at regular 28-day intervals since menarche at the age of 12 years and last for 5 to 6 days. She is sexually active with two male partners and uses condoms inconsistently. Vital signs are within normal limits. Physical examination shows no abnormalities. Which of the following is the most likely cause of this patient's symptoms? Options: A) Endometrial sloughing and uterine contractions mediated by prostaglandin B) Ascending infection of the uterus, fallopian tubes, ovaries, or surrounding tissue C) Fluid-filled sac within the ovary D) Hormone-sensitive smooth muscle tumor of the myometrium Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.