Corn and rice allergies aren't very common and used as a cheap pet food filler for half a century.I also don't understand why we can't have rx diets without grains like corn and rice. I mean youd think they could make it without ingredients that cause rashes and constipation.
I think you're correct about corn. I was generalizing about rice.I have had a few cats that got hives after eating food with corn in it so either its really common or maybe its just the local gene pool? Idk. If you must get perscription food try to get the cans they usualy have less grains and fillers. Canned food is better for cats anyways.