A 67-year-old man with a previous midline laparotomy presents with colicky abdominal pain, vomiting and progressive distension. Initially the pain came in waves, but it has become continuous over the past four hours, and he is now tachycardic with localised guarding. The serum lactate is raised. CT shows a cluster of distended small-bowel loops with two adjacent transition points and reduced enhancement of part of the loop. Which one of the following is the most likely diagnosis?