In my interview, what if they ask me a question on a topic I know nothing about?

The interviewer is rarely interested in your current knowledge. This is why you are applying to University, to expand your knowledge base. It would be useless for you to go to University if you already knew everything so don't panic, it doesn't matter if you don't know something. If a general understanding of the area is required to answer the question then just say to the interviewer 'sorry, I don't think I have come across this before, do you mind explaining a bit about it?' and then using the explanation they give you, start your answer with something like 'well, given what you just told me, I would argue....' Or if it is something that you can try to navigate without knowledge then you can start your answer by saying 'so if I were to speculate......'
The main thing that the interviewer wants to see is how your brain processes things and how you deal with new concepts and ideas. Think out loud and never be too scared to disagree with the interviewer if you don't think what they are saying is correct. Don't just disagree for the sake of it though, they can see right through this as you will not have proper thought behind your disagreement. They are testing how you will respond in a supervision setting so view it as though your teacher is asking you a question in a lesson and show the interviewer how you go about figuring things out.