A 37-year-old woman has noted an excessively heavy menstrual flow each of the past 6 months. She also has noticed increasing numbers of pinpoint hemorrhages on her lower extremities in the past month. Physical examination shows no organomegaly or lymphadenopathy. CBC shows hemoglobin of 14.2 g/dL, hematocrit of 42.5%, MCV of 91 mm3, platelet count of 15,000/ mm3, and WBC count of 6950/ mm3. On admission to the hospital, she has melena and after a transfusion of platelets, her platelet count does not increase. Which of the following describes the most likely basis for her bleeding tendency?