A 34-year-old woman presents with increasing abdominal girth of 3 months in duration. Physical examination reveals a left ovarian mass and mild ascites. The ovarian mass is removed, and the pathology report states "yolk sac carcinoma." Which of the following provides the best serologic marker to monitor the course of disease in this patient after surgery?

Yolk sac tumor is a highly malignant tumor of women under the age of 30 years that histologically resembles mesenchyme of the primitive yolk sac. The tumor secretes alpha-fetoprotein (AFP), which can be demonstrated histochemically within eosinophilic droplets. Detection of AFP in the blood is useful both for diagnosis and for monitoring the effectiveness of therapy. The hormone human chorionic gonadotropin (choice D) is secreted by choriocarcinoma. Estrogen (choice E) is secreted by sex cord tumors.Diagnosis: Yolk sac carcinoma