There's no great way to prepare for on-site clinical questions. Sometimes you luck out and get an anticoag case the month after your anticoag clinic rotation; other times, you can hardly remember your name, let alone which drugs are renally cleared.
If you are given a case to complete and/or present, just think through it logically and do your best. Even if you are struggling for correct answers, if you can organize your thoughts and speak articulately about what you know (and don't know) you will be ahead of the game. Also, the interviewers will likely want to give you feedback after it's over, and part of the process is seeing how you handle constructive criticism.
Just a note: You may or may not be told that there is a case ahead of time, but if the site wants you to give a presentation then you should have ample warning.
In general, be prepared to talk about clinical interventions you have made or patients you have impacted. Even if there is no clinical case per se, questions related to your clinical experience, expertise, and even presentation skills will often come up. I would suggest having 2 or 3 interventions that you can discuss confidently, and also be familiar with some of the presentations you have given. Have an idea of the main point of your presentations in case you are asked what you learned. Remember: If it's on your CV, it's fair game.
