I would try not to worry too much mate! I've been asked a number of questions I didn't know the answer to and just had to concede that "I'm afraid I'm not sure on that one" and whilst it feels pretty rubbish at the time, it's much better admitting what you don't know rather than trying to bulls*** your way through an answer.Being asked a question in a case study partner briefing and having no answer.
If you think about it from a client's perspective, if they're paying your firm a lot of money for advice, they want that advice to be precise and well thought out so it's much better to admit when you don't know something and get back to them later with a well-researched answer! Obviously, this isn't possible in an interview, but I've always found that being honest is appreciated by the interviewer and, chances are, they were only asking you that question to push you and see how far they could get you to go which is normally a good thing.
Hope the rest of the case study briefing went well