Lump on cat spine and bottom

Question
My cat is 12 years old, is up to date with vaccines and suffers from kidney failure but it's not serious at the moment. He currently had a bump in the middle of his spine which isn't hard which has recently broke through the skin, he also has a patch of skin missing near his bottom which is inflamed and is a reddish/brown colour. Overall his behavior is normal and he still has an appetite, however he does hesitate when he jumps on my lap or the bed. He's mostly an indoor cat and as far as I know he hasn't been injured or in any fights.

Answer
The lump could be many things, it is best looked at by a vet. The patch is reddish brown because he is licking it. While you have the vet look at his back have him look at his bottom too. He might have an impacted anal gland.

The lump could be a cyst or a grub in there. Either way, the vet should look it over.