A 6-year-old child complains of frequent liquid stool and vomiting. On the 2nd day of desease the child presented with inertness, temperature rise up to 38,2°C, heart rate -150 bpm, scaphoid abdomen, palpatory painful sigmoid colon, defecation 10 times a day with liquid, scarce stool with mucus and streaks of green. What is a provisional diagnosis?