Hi Marie,
No disrespect or anything but as my intro states I have worked with the public for years in no kill shelters and stuff like that. I have spoken to many people who "self diagnosed" URI, cancer, and any myriad of diseases without taking there animal to the veterinarian. So that is why I asked if this particular cat was taken to the veterinarian.
Also as you know you can get certain antibiotics without taking the animal to a veterinarian.
I am not condoning either of the above scenario's or saying the person did any of the above scenario's. But just pointing out that it is very possible from my experience that the person could have posted the above information without seeing a licensed veterinarian.
Lastly I am a licensed veterinarian technician so I can read

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Hi Heidi n Q,
I just want to say your three ideas for why this particular kitten is not using the litter box sound very possible.