A confusing sigmoidoscopy

This elderly patient with renal disease presented with loose motions and abdominal discomfort. A non-contrast CT had reported a sigmoid mass and a flexible sigmoidoscopy is organised.
WHAT IS THE DIAGNOSIS?

■ Likely diverticular stricture

INCORRECT!

■ Stricture is probably malignant

INCORRECT

■ The symptoms is not due to a stricture

CORRECT!

explanation

This is a benign entero-colic fistula.  It's the explanation for the odd looking "double lumen".  On FoE, 90% got this right.  By the way, we had a similar case in Leeds a few years ago when the unwary endoscopist was puzzled when histology reported the presence of normal duodenal mucosa in what he had reported as "hepatic flexure" ☺ !