Q. All four of my cats have always been perfectly well-behaved. Recently, however, one has been urinating outside the box. I can’t tell which one, and my vet says we need to screen all four for medical problems. Isn’t there another way of identifying the offender?

I’ve been in your situation, so I can tell you one thing for sure: It’s not easy!

For starters, you need to determine the pattern of urination: Is it always on a horizontal surface (your bed, the floor beside the litter box) or is it also (or exclusively) on vertical surfaces (a wall, a piece of furniture)? This crucial bit of information will help your vet differentiate between behavioral and medical causes of inappropriate elimination.

It’s not always easy to tell after the fact, which is why vets recommend a black light (bulbs are available at craft and home improvement stores). There’s something about cat pee that allows a black light to highlight the urine-soiled areas.

So that helps you find the pee, which can give you clues about which cat it is. A surer way is to ask your vet for fluorescein, a dye you can add to a cat’s food that enhances the color of its urine. Offer it to one cat at a time, and eventually you’ll identify the perp.

Want an easier but way more expensive solution? Buy one of those motion-activated video systems, and you can actually see who’s doing the damage.

Dr. Patty Khuly has a veterinary practice in South Miami and blogs at www.dolittler.com. Send questions to khulyp@bellsouth.net, or Dr. Dolittler, Tropical Life, The Miami Herald, 1 Herald Plaza, Miami, FL 33132.