The vet honestly doesn't know the answer to your question.
I'd suggest this is likely. Assuming both ailments are relatively common and not obviously known to be rare, I'd bet the vet just doesn't know the data necessary to discuss base rates in a meaningful way that would help determine X or Y.
Side note: My experience is that sometime the tests needed to help narrow down illnesses in animals are prohibitively expensive.
If it's worth saying, but not worth its own post, then it goes here.
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