A 14-year-old nulligravid female presents to the emergency department with the complaint of lower abdominal pressure and pain. The abdomen is nontender on examination, but you palpate left lower quadrant fullness and a Chapman tender point at the left sacroiliac junction. Pelvic ultrasound reveals a normal pelvis except for an 8-cm complex left adnexal mass. There are multilocular cystic areas as well as some areas of hyperechogenicity. There is no free fluid in the cul-de-sac. The most likely diagnosis is