A 24-year-old woman, a kindergarten teacher, has been sick for 2 days already. Disease onset was acute. She presents with elevated body temperature up to 38.0°C, pain attacks in her lower left abdomen, liquid stool in small amounts with blood and mucus admixtures 10 times a day. Pulse – 98/min., blood pressure – 110/70 mm Hg. Her tongue is moist and coated with white deposits. The abdomen is soft, the sigmoid colon is painful and spastic. Make the provisional diagnosis