An 18-year-old male presented with diarrhea yesterday, passing loose stools 5-6 times. Today, the stools became mucopurulent, occurring every 2 hours with small volume, accompanied by tenesmus. Physical examination revealed a soft abdomen, tenderness in the left lower quadrant, and hyperactive bowel sounds. Laboratory tests showed WBC 14.6×10<sup>9</sup>/L (86% neutrophils), stool microscopy revealed RBCs 20/HPF, WBCs covering the entire field, and 0-1 macrophages/HPF. Multiple classmates had similar diarrhea in the past week. The clinical diagnosis is bacterial dysentery. In China, the most common causative pathogen of this disease is: