Pain in the abdomen above the root of the left thigh, corresponding to the sigmoid colon, is most often caused by colitis, which occurs more often in young people and is also an autoimmune disease. The pain can be triggered by irregular diet, overeating, alcohol consumption, and eating raw, cold, or hard foods. Patients may also have abnormal bowel movements, mainly bloody or purulent stools, repeated episodes of diarrhea, and many bowel movements per day, and should undergo timely colonoscopy, which will also reveal congestion, edema, and ulcer formation in the intestinal mucosa. After the diagnosis of ulcerative colitis is confirmed, attention should be paid to general treatment and diet, and the intake of calories and protein foods should be ensured, and medication should be administered if necessary.