A 20-year-old female presents with persistent fever for 5 days, facial edematous skin lesions, swollen and painful knee and ankle joints, lower extremity edema, and scattered petechiae. Laboratory tests: ESR 100 mm/h, hemoglobin 80 g/L, reticulocytes 10%, Coombs test (+), platelets 40×10<sup>9</sup>/L, urinalysis: protein (++), RBC 5-10/HP. The most likely diagnosis is: