Defecating outside of litter box

My mother had a cat(I now have the cat) who defecates outside of the litter. She is about 14 years old and is in great health besides her being a little on the obese side. The vet says she does this because she is getting old and somewhat senile.

Mookie now lives with me because my mom was going to have her PTS. I wouldn't allow that as she has been in our family for many years and even though she has quite the attitude at times, I love her dearly. I also don't believe that she isn't using the litter box even though my Mom's husband said he caught her pooping on the floor. My mom had just put another senior cat to sleep because she thought this cat was not using the litter box. Three days later, they found out it was my grandma's cat not using the litter box!!! My grandma's cat has issues with my grandmother so I know exactly why she doesn't use the litter.

Mookie has been with me three days and no accident yet. I was just wondering........has anyone else had a SENIOR cat not use the litter all of a sudden???

Aometimes, senior cats have a hard time to make it to the litter box in time - even though they do try. Or their arthritis makes it hard for them to jump in and out of an especially high box? PIf this is the case- and it may not be - hard to say offhand - you could place litter boxes in strethic locations aroound the house and show her where they are.

Once I had a kitty that got frightened at his usual potty spot and then just seemed to try different locations. It eventually passed. I do wonder about kitty reasoning ability, perhaps if it hurts to go to the litter box kitty might try a different spot because she is associating the litter box with pain? I'm not sure if it works like that but thought I would mention it just in case.