A 29-year-old female is evaluated after thrombocytopenia (38K/mm3) was noted on preoperative laboratory testing for an elective bilateral tubal ligation. Eight months ago, she delivered a healthy baby; postpartum examination showed the patient was healthy, on no medications (including OTC drugs), and had a documented normal platelet count. The CBC was otherwise normal at that time, as was the coagulation profile and renal and liver function tests. Anti-nuclear antibodies were negative and anti-platelet antibodies were positive. Serologic testing for HIV, EBV, CMV, and a hepatitis panel were negative. Currently, she confirms a 1-month history of bleeding gums when brushing her teeth, but denies bruising. Her menstrual cycles have been light and short in duration. She takes oral contraceptive pills. The physical examination is unremarkable. No rashes, cutaneous lesions, or lymphadenopathy are present. What is the most likely diagnosis?