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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
Sorry for the wait. I was typing and then got called away.
It sounds like some type of bladder tumor to me as well. It could be inflammatory cystitis, but that is usually uniform around the bladder vs occurring in just one area. The trigone area is the typical location for a TCC (transitional cell carcinoma). Lymphoma can form anywhere.
I would be a little hesitant to do a biopsy. The reason is that TCC tumors can be spread during a needle aspirate or biopsy procedure. It's better to go in with a scope or surgically to get the sample so you don't seed tumor cells elsewhere in the abdomen.
If you just decide to forego diagnostics and start treatment, it's not at all the wrong decision. I would go with a steroid like Prednisolone. If it's Lymphoma, it can put it into remission. If it's TCC, it can help slow down the spread some, but it's not likely to do much other than improve her quality of life.
I hope this helps, let me know if you have any other questions.