"Base rate" is statistics jargon. I would ask something like "which disease is more common?" And then if they still don't understand, you can explain that its probably the disease that is most common, without explaining Bayes rule.

So, it seems like there could be 2 things going on here: 1: Maybe, in your Vet's mind, she is telling you "We can't tell if this is A or B", and you are asking "But which is it?", and by refusing to answer she is doubling down on the whole "We don't know A or B" situation. Like, I know what you mean is what you actually said, but normal people don't say that, and the Vet is trying to reiterate that you do not know which of A or B this is. She is trying to avoid saying "mostly A", and you saying "ok, treat A", and then the dog dies of B, and you are like "You said A, you fraud, I'm suing you!". 2: The vet honestly doesn't know the answer to your question. She is a person who executes the procedures in his/her manuals, not a person who follows the news about every animal's frequent ailments. In her world if an animal shows A you do X, if an animal shows B you do Y. Your question is outside of her realm of curiosity. As far as another way to phrase this, I'd go with "Well, which do you think it is, A or B?". The vet's answer ought to be informed by her experience, even if it isn't explicitly phrased as "well, mostly this is what dogs suffer from". If she reiterates that there is no way to know, I'd figure this was a first case, CYA situation, and stress that I wouldn't be mad if she was wrong.