A 38-year-old man complains of a spasmodic abdominal pain, frequent liquid stool with mucus and fresh blood admixtures. He has been suffering from this condition for 2 years, during which he has lost 12 kg. Objectively: Ps —92/min., blood pressure — 100/70 mm Hg, body temperature — 374 °C. The abdomen is soft and painful along the large intestine. The sigmoid colon is spastic. In the blood: erythrocytes — 3.2 × 1012/L, Hb — 92 g/L, leukocytes — 10.6 • 109/L, ESR — 32 mm/hour. Irrigoscopy shows narrowing of the large intestine, no haustra, blurred margins, "lead-pipe" sign. What is the most likely diagnosis?