Choriocarcinoma

A 33-year-old woman, gravida 2, para 2, comes to the clinic because of shortness of breath and coughing up blood for the past two weeks. She gave birth to her second child two months ago, and has been breast feeding. She has never smoked tobacco. She has also experienced unintentional weight loss, heat intolerance, and anxiety. Her temperature is 37.0°C (98.6°F), pulse is 96/min, respirations are 18/min, and blood pressure is 142/86 mm Hg. Physical examination shows labored breathing and mild exophthalmos. Laboratory studies show an elevated beta-human chorionic gonadotropin. Pelvic ultrasound shows a heterogeneous mass invading into the wall of the uterus. A biopsy is performed and shows highly vascular placental tissue without chorionic villi. Which of the following is the most likely diagnosis?