What is the key finding?
Large small bowel mesenteric mass. This mass has appearances of a carcinoid, although the classical desmoplastic reaction is missing. Differentials could include lymphoma (less likely to be mass like, and there is not widespread lymphadenopathy) or a large single metastasis.
What is the cause of the patient's initial diarrhoea?
Carcinoid tumours can present with paraneoplastic carcinoid syndrome, with diarrhoea being a common symptom.
Does the mass account for the patient's other symptoms of abdominal distension and subsequent constipation?
Yes, there is a partial small bowel obstruction.
Large mass in the small bowel mesentery with enlarged lymph nodes surrounding it. The anterior aspect of the mass is indistinguishable from a segment of mural thickening of the mid ileum in the right iliac fossa, which forms a focal transition point between dilated proximal left relatively collapsed distal small bowel. Contrast does pass beyond the transition, consistent with a partial rather than complete obstruction.