A 2,5-year-old child fell ill 10 hours ago, became anxious, held on the stomach, had fever up to 38 degree С. In 7 hours condition has worsened with vomiting and diarrhea. Physical examination: tongue is dry, coated. Abdomen barely participates in breathing. Palpation is impossible due to childs anxiety. Digital rectal investigation reveals overhang of front rectum wall. What is the most likely diagnosis?