Cat ignoring litterbox

Right before typing this post: Not more than 10 minutes after cleaning her box, she defecated on the left side and urinated on the right side. Type of behavior has been continuing for months. We've tried all sorts of boxes and types of litter. Cat is geriatric (16.5 yrs). Low sided boxes result in cat not being able to assume a squatting position in the box and getting an IUD or having her rear end hang off the edge of the box while she urinates and defecates. The top-loading sweater tub type box is the only solution that works. It worked for several years. However, now she refuses to use a box at all. We have provided a step up into the box. The box is on the main floor of the house. She is not constipated and does not have kidney problems (confirmed by blood sample). Please advise. This must be some sort of behavior problem, right? How do we encourage her to use the box? We're getting tired of cleaning up after every day. Thank you in advance for any insight you have.

Following up with more information: She is the only cat in the household. A second litterbox of the same type exists but is no longer used because it is in the basement and the cat's arthritis makes going up and down the stairs uncomfortable.  She displays no signs of anxiety and no notable events preceded this behavior.

Well, it is a behavioural problem insofar as this behaviour is causing you problems.   But the solution relies on understanding what is happening.  It sounds as though your cat wants to go in that place / location, because she is going by the side of the box, so my immediate guess is that maybe it is the box which is a problem.  HYPOTHESIS A:  I wonder whether the smell of the cleaning stuff is putting her off?   If it smells like an invader / chemical, she may well not want to mark directly on it.   Sometimes, if you can bear to, leaving a little of the cats own faeces in there can be helpful, to help mark the territory as hers.   HYPOTHESIS B:  The other point that you raise, is that your cat may have arthritis, and may associate squatting in her box with pain, which would sopn put her off.   In this case, perhaps your vet can offer treatment for feline arthritis.    I am struggling to picture the litter boxes that you describe, but it sounds to me as though ease of getting into the tray might be important - perhaps a none slip surface, wide box, and litter that is not painful to put her feet on, may help?    We would reccommend discussing this with your vet, who may also be able to assist you in finding places to put the litter trays.  It is recommended that cats have a litter tray per cat plus a spare, but in this case I don't think that the spare counts, as the cat is unable to access the cellar.   Is there another possible place?  Cats are notoriously fussy abput toileting sites and it is brilliant that you are trying to address the problem for her.

Kate has written an awesome article in our site blog, which you might want to check out as it covers some ideas that I didn't suggest here.   You can type key words into the blog - search for: 'Why is my cat not using the litter tray?'