Your vet must have noticed that his urine wasn't as concentrated as it should be, and that's why she mentioned early kidney failure. I've learned not to panic when I hear those words since I've had five cats over the past several years with that condition and I only lost one within a year. At one time, I was giving at least three cats sub-q fluids every evening after work. It wasn't fun, but it's amazing what that treatment can do for a cat. Of course, if this does prove necessary for Rusty, given his rather belligerant personality, he may need to be kept on a mild sedative until he gets used to it.

However, here's hoping that it's just another bladder infection.