What are the main differential diagnoses for this appearance?
Fulminant Crohn disease - Crohn disease usually presents with patchy involvement of the colon but these can become confluent if severe. The following also need to be excluded; Infection - especially with enterohaemorrhagic infection (shigella, salmonella and E.coli) ; ischaemia - especially in elderly patients or those with clotting disorders; radiation colitis.
Segment of colon showing; extensive congestion and haemorrhage, irregular ulceration with pseudopolyp formation, and only minimal wall thickening.
The macroscopic features favour ulcerative colitis; this is based on continuous ('diffuse') ulceration, pseudopolyps (they are not true polyps, rather they are islands of residual intact mucosa within an area of ulceration), the muscualris propria and serosa are unremarkable (c.f. Crohn disease).