"That is an excellent question! And I have absolutely no idea!"
"In my 10 years of practice here, I have never heard anyone ask me that question before!"
"I really don't know. But if you give me one second, I can look it up for you."
"I'm going to have to think about this one. Give me until our next appointment to sort this out. I want to do some research on this."
"If you don't mind, I am going to have my partner look at this also. Sometimes two minds are better than one."
"That is not something that I am good at evaluating, if you don't mind, I'd like to refer you to a specialist to see if they can help us figure this out."
"I'm not sure what is causing your symptom, but I will do my very best to help you find out. We will make of list of some possibilities and start crossing them out as the tests come back."
Over the last ten years, I have found that I have needed to say these things less often. But it's still good to remember that no matter what questions may come up, simply saying "I don't know" is nothing to be ashamed or embarrassed about. On the contrary, I have found that when I am brutally honest about my ignorance, patient will trust me more about the things that I do know.