What's difficult for one person might be easy for another. A tough customer might be more of a challenge than chasing a cat around the office. Think of an example. Consider framing your answer like this:
"When I volunteered at the animal shelter, I had a dog that had vomited and pooped all over his cage and rolled around in it. I cleaned him up and his cage. He was a large Rottweiler mix and very sick so it was a challenge to move him as well."
To show your enthusiasm and motivation, you could follow up with your statement by saying, "I understand the challenges of being a vet assistant, but it's worth it to me. I know this experience handling animals will help me reach my career goals."
What keeps you motivated when you have to do the dirty work? Time to think about how this job fits into your career goals!