A 28 year old women has a 3-month history of easy bruising and bleeding gums. She feels otherwise well. Medical and family histories are unremarkable, and she takes no medications. On examination, vital signs are normal. Petechiae are present on the buccal mucosa and pretibial areas. No lymphadenopathy or splenomegaly. Hemoglobin 8.4 g/dl Absolute 68 (normal 60- reticulocyte count 100) Leukocyte count 2300/ul Direct Negative antiglobulin (coombs) test Platelet count 18,000/ul A peripheral blood smear shows platelets are decreased in number but otherwise normal. Bone marrow examination shows hypoplastic marrow (decreased cellular element) with normal maturation. Which of the following is the most likely diagnosis?