R. Hershel Schachter writes in this week's TorahWeb (link):
There is a terrible misconception that the laws of Shabbos do not apply to doctors. This is absolutely incorrect. No profession exempts anyone from any mitzvos. Medical students are certainly not exempt from Shabbos observance. And even after having completed his school years, the future doctor must take special care to make sure he has a Sabbath-observant residency. If this can not be arranged, the student must simply look for a different profession.On the other hand, some doctors will argue that while this is absolutely correct, any residency can be made "Shomer Shabbos" if you know how to do it. See this post and the comments.