PLE-4584

A 32-year-old woman, gravida 2, para 2, comes to the physician because she has been amenorrheic for 4 months. Examination shows a well-estrogenized vagina and no evidence of virilization or other abnormalities. A serum pregnancy test is negative. She is given medroxyprogesterone and has the onset of bleeding 3 days later. What is the most likely cause of her condition?