A 15-year-old male patient developed low-grade fever and rhinorrhea one week ago after catching a cold. After self-administering 'Ganmaotong', he experienced abdominal pain, diarrhea, and bloody stools. The next day, scattered purpura appeared below the knees of both lower limbs. Lab results: Hb 120g/L, WBC 12×10<sup>9</sup>/L (neutrophils 0.8), PLT 160×10<sup>9</sup>/L, urinary protein (+), RBC 10-20/HPF, WBC 5-10/HPF, granular casts 1-2/HPF. What is the most likely diagnosis?