Most probably he will be asked some basic stuff that pianist and doctor must know. Like, how many black buttons there are on piano? Doctor is quite wide category so that might depend on what type of doctor he is. Question might be about number of bones in human body, average weight of 12 years old boys and etc. Actually it doesn't matter too much what question is and it does not even matter if you know answer. It is completely possible that you don't understand officer's questions because of how he/she speaks - the most important thing is your reaction and that's all. Those people are trained to identify suspicious behavior and unnatural nervousness in your side.