A 26-year-old female presents with recurrent purpura on both lower limbs and increased menstrual bleeding for 1 year. Spleen is just palpable below the costal margin. No prior medication history. WBC 9.6×10<sup>9</sup>/L, Hb 101g/L (anemia), PLT 24×10<sup>9</sup>/L. Bone marrow shows active proliferation with 75 megakaryocytes. ESR, urinalysis (UA), liver and renal function are normal. The most likely diagnosis is: