Has the cat been de-clawed? If so, many refuse the litterbox after this procedure.
Could it be a territorial issue with the other cats? Did you recently acquire a new cat? Or maybe they all got along fine when they were young but now, as this one matures, she is establishing her territory and/or feels threatened by the other cat(s)?
Is the litter box comfortable, easily accessible and in a quiet area? Do you have 2 boxes? With 3 cats you may want to consider a second box.
We had a neutered Maine Coon who used to do this, especially with laundry on the floor. We had to put him in his box in the morning and at night to 'go.' When he went we praised him; if he didn't, and jumped out we put him back in again. He'd go. Like your cat there was no infection or illness. (He was older when we adopted him so I think he was a bit lazy. LOL)