An 18 y.o. girl complains of weakness, dizziness, loss of appetite, menorrhagia. There are many-coloured petechiae on the skin of the upper extremities. Blood test: Hb- 105 g/l; RBC- 3,2 × 1012/l; C.I.-0,95; thromb.- 20 × 109/l. The sedimentation time according to Lee White is 5'; hemorrhagia duration according to Duke is 8', "pinch and tourniquet"test is positive. What is the most probable diagnosis?