"We're fostering cats for the local rescue. This one has some medical needs, so we thought it easier for another of the fosterers to care for it while you're in town" - what I'd say. What I'd be thinking "and since you have no idea of what a cat's medical needs could be, this is easier for everyone!"

My sister is a temporary cat foster for their local rescue. 😺😇 I have a rescue dog. :-)