K.B.
I cannot comment on your cat's diagnosis and whether or not it was correct. I will say that, like others have pointed out, sometimes we have a list in our heads of what it COULD be, and sometimes we will start out conservatively and see how it responds to treatment. Medicine (including veterinary medicine) isn't as exact as everyone seems to think it is. It's not like fixing a car. If a pet is not better, just like if a person is not better, they need to be rechecked and re-evaluated, and then a different treatment plan may be discussed. When I had gall stones, my doctor didn't think it was gall stones right away - she figured it was acid reflux and tried me on super-strength Tagamet. When I was still having problems, she said first she would send me for an ultrasound, then other imaging studies if the ultrasound didn't show anything (I was having bouts of severe abdominal pain). Turns out the ultrasound showed the gall stones and it was not what my doctor was expecting, given that I was young and of normal weight. I didn't get upset at my doctor - because I get where she's coming from, it's what I deal with every day. If I told every client with a sick pet right off the bat that their pet needed bloodwork, x-rays, an ultrasound and possibly surgery and biopsies, they would freak. It just depends on the situation - some yes, but sometimes conservative treatment works and ends up saving owners money in the long run. Sometimes we can tell right away what the problem is...sometimes we have a strong suspicion on what most likely the problem is...and sometimes we have to make our best educated guess, or get more information.
Believe me, I would love nothing more than to get it perfectly right the first time just with an exam and never have people have to come back or consider further diagnostic tests. But it just doesn't happen that way. We are human, doctors are human, and we can't be all-seeing all-knowing all the time.