A 38-year-old woman has become increasingly fatigued for the past 3 months. During the past week she has noted purple blotches on her skin. On physical examination there are purpuric areas of skin on her trunk and extremities. She has no hepatosplenomegaly and no lymphadenopathy. Laboratory studies show Hgb 6.8 g/dL, Hct 20.7%, MCV 91 fL, platelet count 28,760/microliter, and WBC count 1940/microliter. Which of the following is the most likely diagnosis?