A 32-year-old woman complains of increasing spastic pains in her lower abdomen that occur after emotional stress. Bowel movements are intermittent: 2-3 defecations after waking in the morning alternate with constipations that last for 1-2 days. Objectively body mass is retained, palpation of the sigmoid colon is moderately painful. Hb- 130 g/L, leukocytes – 5.2 g/L, ESR - 9 mm/hour. Rectoromanoscopy is painful due to spastic condition of the intestine, intestinal mucosa is without changes. Intestinal lumen contains large amounts of mucus. What is the most likely diagnosis in this case?