The first I time I had ever heard of Feline Urological Syndrome was back in 1990 when I noticed one of my male cat's trying to urinate in the corners of rooms while crying as if he were in pain. At the time, we had four other indoor cats so I was not aware of exactly which cat urinated in which litter box.
We immediately took him to our vet for an examination and he was quickly diagnosed with Feline Urological Syndrome and kept at the animal hospital for a day in order to have a catheter inserted. Feline Urological Syndrome can be brought on by several factors; however, the most common cause is a urethral blockage due to crystallization. Cat food containing high ash content, fillers such as wheat, soy, and corn which cause the crystallization are extremely dangerous and can kill your cat within hours if not caught in time!
The seriousness of this medical condition can not be overstated! If your cat is diagnosed with this, they will need to be fed a special Urinary Diet food prescribed by the vet for the rest of their natural life.
This problem is more common in male cats than females. The reason for this is because males have a much narrower urinal tract than females. Our cat Leo has F.U.S. and once a week we must perform what I refer to as a "potty check". Meaning, I give him a completely fresh litter box using only the clumpy cat litter and separate him from the other cats by putting him in the catio after his dinner. This is imperative for keeping track of his urinary habits.
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I have included a couple of links for further reading:
https://felineinstincts.com/feline-urological-syndrome-fus
https://www.catster.com/cat-health-care/feline-urological-syndrome