A 35-year-old woman is evaluated for a long history of easy bruising. The peripheral smear shows only a few, large, young platelets, while other cell lines are normal. Marrow studies show increased megakaryocytes. Which of the following is the most likely diagnosis?

This clinical scenario is most consistent with Idiopathic thrombocytopenic purpura, which is an immune-mediated destruction of platelets by the spleen. The peripheral smear and marrow show the features described. Idiopathic thrombocytopenic purpura is distinguished from microangiopathic hemolytic anemia and thrombotic thrombocytopenic purpura by the absence of fragmented red cells. In both thrombasthenia and von Willebrand's disease, the platelet count will be normal, although platelet function is impaired. Ref: Diamond B., Lipsky P.E. (2012). Chapter 318. Autoimmunity and Autoimmune Diseases. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.