What is the cause of this patient's presentation, and what is the underlying reason for this?
Large bowel obstruction secondary to a distal descending / sigmoid colon mass.
What is the most likely cause of the obstructing mass?
This has an apple-core appearance almost certainly representing a colorectal adenocarcinoma.
True or false: there is intramural gas, indicative of superimposed ischaemic colitis.
False: Widespread gas is seen adherent to the bowel wall. It can be shown not to be intramural, as A) the gas is on the luminal side of the mucosa and B) it is only visible where there is liquid faecal matter next to it: it is never seen anti-dependently. Also the fact that the bowel wall enhances normally.
True or false: the fact that the small bowel is not dilated should not dissuade you from the diagnosis of a large bowel obstruction.
True. This represents a competent ileocaecal valve, which continues to allow small bowel content into the large bowel, but does not allow retrograde flow from caecum into small bowel. Eventually the small bowel would not longer be able to empty properly and would also start to distend.
There is distension of the large bowel proximal to the sigmoid, with prominent fluid levels. An obstructing mass in the upper sigmoid / distal descending colon with contrast enhancement is noted, the large bowel beyond it being collapsed. In addition the ileocaecal valve is competent and despite significant distention of the large bowel, the small bowel is collapsed.