A 28-year-old woman. One year ago, she experienced left-sided abdominal pain accompanied by diarrhea, with 4-5 bowel movements per day, loose stools containing mucus and small amounts of pus/blood, but no fever, poor appetite, or vomiting. She self-administered furazolidone for 3 days, and symptoms resolved after discontinuation. Since then, bowel frequency increases with cold exposure or dietary indiscretion. This time, symptoms recurred after exertion. Stool microscopy: WBC 10-15/HP, RBC 5-6/HP. The most likely diagnosis is: