I don't understand how in the US they could possibly ask this question. This is an encroachment on the primary constitutional rights of the individual. This is outright discrimination. I'm going to try not to get up on my soapbox so I will stop there.

I also don't think it would be possible for them to find out or prove that you are bipolar, unless you tell them, due to the doctor patient privilege. Unless you allow your doctor to release the information they can't. That is unless you are a danger to yourself or others so I guess if you were planning to run somebody over you are in trouble.