A 33-year-old female presents with pain in the lower abdomen. She says the pain has been ongoing for the past 2 months and she feels full in the lower pelvic area. At times, she has had difficulty holding urine and has been constipated. X-rays and bloodwork are unremarkable. Needle aspiration of the mass reveals clear fluid from the left ovary, which measures 7 cm. The fluid has no evidence of malignancy. She most likely has which of the following conditions?