Is it allowable noy to fast in Ramadan for one day in which I have an 8 hour exam that can not be postponed and is critical for my entire future career? I have to pass this exam to obtain my license to practice medicine in USA. The exam starts at 10:30 am and ends at 6:30 pm.
Please advise me.

Honorable brother,
It is only permissible to refrain from fasting in case of traveling. According to the opinion of the Hanafi scholars, the maximum duration that allows a traveler to break fasting in his country of destination is to stay there for 15 days. If the case is not like that with you, you must observe fasting. However, if it is difficult for you to fast, it is permissible to break fasting on that day and make it up later after Ramadan.
May Allah help and grant us all success.