Recently, I opted for interviews with 2 MNCs. They asked me about my preferable topics and I believe this is a general trend (asking about your comfort zone). Below are few common topics you can concentarte on:-
Activity Methods, (If you are working in Pega7, then what are those alternatives)
UI related topics, like "Source of a Repeat Grid", "Source of Drop-down menus" etc..
Flow, Flow Action, Different flow shapes (simple shapes)