A 31-year-old woman has had a persistent fever for the past 2 months. Her temperature has ranged from 38.3degC to 38.6degC on multiple occasions. On physical examination, she has diffuse abdominal pain and mild splenomegaly, but no hepatomegaly or lymphadenopathy. Laboratory studies show Hgb, 13.2 g/dL; Hct, 39.8%; MCV, 930 mm3; platelet count, 242,000/ mm3; and WBC count, 12,290/ mm3 with 71% segmented neutrophils, 19% lymphocytes, and 10% monocytes. CT imaging of her abdomen shows an ill-defined pelvic soft- tissue density mass with a mottled lucent center and a small, square, radiopaque area. Review of her medical record reveals that a salpingo-oophorectomy for ectopic pregnancy was performed on the left side 3 months ago. Which of the following is the most likely cause of her persistent fever?