An autopsy of the body of an elderly man, who was suffering from acute intestinal disorder during his last 2 weeks of life, has revealed the following change in the rectum and sigmoid colon: brown and green film covering the mucosa is detected. The itestinal wall is thickened; the cavity sharply narrows down. Microscopy reveals mucosa necrosis of varying depth, necrotic tissue is pierced through with fibrin threads, leucocytic infiltration is observed. What diagnosis is the most probable?