OH MY GOSH
I just looked up his other meds Baytril the antibiotic an dits side effects can cause crystals in the urine, Thats what he just had the surgery for.
The use of enrofloxacin can produce crystals in urine. These crystals may show up on a laboratory test thus it is important to be aware of this side effect.
See retinal damage below regarding the recently described feline retinal problem associated with enrofloxacin at higher doses.
http://www.veterinarypartner.com/Content.plx?P=A&A=553