A 40-year-old woman has noticed progressive enlargement of the abdomen over the past 5 months, although her diet has not changed, and she has been exercising more. Physical examination shows no palpable masses, but a fluid wave is present. Paracentesis yields 500 mL of slightly cloudy fluid. Cytologic examination of the fluid shows malignant cells. An abdominal ultrasound scan shows a 15-cm multilobular mass that involves the right adnexal region. The uterus is normal in size. The mass is surgically removed; the figure shows the gross features of a section of the excised mass. What is the most likely diagnosis? A. Choriocarcinoma B. Dysgerminoma C. Granulosa cell tumor D. Mucinous cystadenocarcinoma E. Teratoma with malignant transformation Mucinous tumors of the ovary are of epithelial origin, are less common than serous tumors, and tend to be multiloculated. The appearance of ascites suggests metastases, which is most common with surface epithelial neoplasms of the ovary. Choriocarcinomas rarely reach this size because they metastasize early; they are typically hemorrhagic. Granulosa cell tumors and dysgerminomas tend to be solid masses. Teratomas are germ cell tumors differentiating into three germ layers; malignant transformation is rare, and is usually an element of squamous cell carcinoma from the ectodermal component.