All UC involves the rectum since that's where it starts. It travels upward in a continual pattern ... to limited or throughout.
The new doc I'm seeing told me there are some rare cases where the left side is affected and an isolated spot somewhere over on the right is affected... but it apparently doesn't happen very often.
When I was initially diagnosed years back, my inflammation pattern looked like classic UC. But now, I'm told the left side is healthy and the trouble spot is at the turn between the ascending and transverse sections... looking more like Crohn's or possibly colon cancer. I've always had pain and discomfort on the right. Docs have suspected appendicitis twice, but the appendix has been normal.
I mentioned this in another post, but my doc said research is leading them to believe there is some overlap in Crohn's and UC and they're not such totally separate entities. It's my personal opinion it's all a continuum... from irritable bowel on to the more serious IBDs.