A 30-year-old woman is evaluated for 2 week history of easy bruising and epistaxis. she had previously been well and take no medication. on examination, vital signs are stable. she has numerous petechiae on her lower extremities and ecchymosis on her legs and arms. no lymphadenopathy or hepatosplenomegaly is noted. her laboratory investigation show hemoglobin 12.5 g/dl , white blood cell count 5700/uL, platelet count 10,000/uL , and normal metabolic screening. the peripheral blood film shows decreased platelet and several large platelet. which of the following is the most likely diagnosis?