A 26-year-old woman complains of cramping abdominal pain, diarrhea with a significant amount of mucus and blood, and fever of 37.5-38.0°C. Objectively, her skin and mucosa are pale, the body type is asthenic. Palpation detects pain along the large intestine. Colonofibroscopy reveals edematous wall of the rectum and sigmoid colon erosions, small ulcers, and mucus with blood in the lumen. What is the likely diagnosis in this case?