I started out thinking, well, you're going to have to defend it to others anyway, right? So why not just assume this person is legit and respond accordingly. But the more I think about it, the more I think it's not so much that she wants you to defend it, it's the way she is saying it.
I mean, she could have questioned your topic by saying, "why did you pick (topic)? Did you consider that (topic) whatever whatever?" In other words, give you reasons why she doesn't like the topic or otherwise present an intelligent question. I think the "I'm sorry but (topic)?" just makes me question her motivation.
Is she trying to help you or is she just trying to make you look bad or make herself look better than you?
I probably would respond with encouragement to ask me more specific questions or let me know if she sees a specific problem with my topic... in other words, I'd try to get a more intelligent comment/question out of her. If she replies well, then continue the conversation. If not, I'd probably just move on.