An 18-year-old male presented with diarrhea (5-6 loose stools yesterday) progressing to mucopurulent stools every 2 hours today, scant volume, accompanied by tenesmus. Physical exam: soft abdomen, left lower quadrant tenderness, hyperactive bowel sounds. Laboratory tests: WBC 14.6×10<sup>9</sup>/L (86% neutrophils), stool microscopy shows RBCs 20/HPF, WBCs covering entire field, macrophages 0-1/HPF. Multiple similar diarrhea cases in class this week. Clinical diagnosis: bacillary dysentery. Which intestinal site has the most severe pathological changes?