Can I use a run-time variable (for instance a choice from a dropdown list) in a question object (text fill-in-blank) to check the answer?

If you want to use built-in question objects, then I don't understand how it can be made. If you want to build custom fill-in-blank question, then the answer is yes. If you want to use run-time variable in user interactions then checking the answer (i.e. inserting corresponding action sequence) becomes your responsibility.