救急外来に日本語の話せない68歳の外国人男性が来院した。対応した臨床修練外国医師が診察と検査を行い記載した診療録の一部を示す。

Presenting complaint:
Abdominal pain at the left lower quadrant.

History of presenting complaint:
Sudden onset of sharp pain at the left lower abdomen 3 days ago.
Associated with nausea and chills.

Examination:
Temperature 37.2℃.
No pallor or jaundice.
Generalized abdominal distension with abdominal tenderness and localized rebound tenderness at the left lower quadrant. Bowel sounds are reduced.
No tenderness or mass on rectal examination.

Investigation:
WBC count: 11,300/μL, CRP:9.8mg/dL
CT:multiple small pouches with thickened bowel walls of the sigmoid colon.

　診断はどれか。