A 18 y.o. woman complains of weakness, dizziness, loss of appetite, menorrhagia. There are petechiae on the skin of the upper extremities. Blood test: Hb- 105 g/L; RBC- 3,2 × 10i2/L; coloured index- 0,95; thromb.- 20 × 109/l. The sedi-mantation 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?