Q. My senior cat won't use the litter box she will go to the bathroom in front of it or beside it my other 2 use it just fine. What can I do
DR. ANDREA M. BRODIE, DVM
A. Your senior kitty may have arthritis pain that keep her from squatting in the box as it would be painful. Another reason could be that she wants to use the box, however the litter feels bad or smells bad to her (perfumed, or not cleaned out often enough). There should be one more box than cats in the household. Sometimes cats mob each other when going potty , so that would be something to watch for.
You should also have your cat examined by a vet as urinary tract infections can also cause such behaviour.