I temporarily got caught in the Sad and Useless website, and found Science is Fun.

Most of the claims seem plausible, but I can't guess why cat urine would fluoresce.

Comments have directed me to Why Does Urine Glow Under a Black Light? which says:

You can use a black light to detect body fluids. It's actually a good way to look for pet urine or make sure a bathroom or hotel room is really clean. Cat urine, in particular, glows very brightly under ultraviolet light. Urine glows under a black light primarily because it contains the element phosphorus. Phosphorus glows yellowish green in the presence of oxygen, with or without black light, but the light imparts additional energy that make the chemiluminescence easier to see. Urine also contains broken down blood proteins that glow under a black light.

Certainly other mammals including humans have both phosphorus and broken down blood proteins in their urine, so is the proposed specificity to cat urine just an issue of concentration perhaps?

www.sadanduseless.com/science-is-fun/ cat pee glows under black light

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    $\begingroup$ I don't find it implausible, but why cat urine specifically, and not of other animals? $\endgroup$ – John Dvorak May 21 '19 at 4:53
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    $\begingroup$ I actually think thoughtco simplifies quite a bit. I don't think there is pure P in urine. $\endgroup$ – Roland May 21 '19 at 14:11
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    $\begingroup$ Could possibly be because of Urobilin. Also, diet also influences the urine composition. Some of the excreted products can be fluorescent (for e.g. riboflavin) $\endgroup$ – WYSIWYG May 21 '19 at 14:24
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    $\begingroup$ Cat is a carnivore. Myoglobin is also a heme protein and its breakdown would also produce the "bile pigments" and their downstream excretion metabolites. It is possible that a carnivore might produce more bile pigments because of their diet, compared to a herbivore. However, I am not sure about it. $\endgroup$ – WYSIWYG May 21 '19 at 14:29
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    $\begingroup$ @John Dvorak: In fact, other sorts of urine, including human, fluoresce under UV light. Don't happen to have a cat, so I can't check relative intensity :-) $\endgroup$ – jamesqf May 22 '19 at 2:43

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