A 75 year old female presents to the doctor with a chief complaint of vaginal spotting. She has been postmenopausal for 25 years and does not take hormones. An ultrasound is performed, and shows a mass in the uterine fundus. A hysterectomy is performed, and pathologic examination of the removed uterus reveals a malignant tumor of the endometrial glands and stroma. Which of the following is the most likely diagnosis?