I heard somewhere that "For the MD part, getting in is the hard part, for the PhD part, getting out is the hard part." Honestly, it is probably one of the best way to describe a difference between MD and PhD programs. People go through all sorts of hoops to get into medical school, but once you are in, schools will do their best to get you out with a degree unless you are just hopeless.
However, PhD programs might seem easier to get into compared to MD programs, but once you are in, it is extremely difficult to get out with a degree. PhD training is a screening process itself that is incomparable to anything else. You just simply can't predict how well someones is going to do in a lab from an application or a standardized test.
Here I am supposedly a few months away from graduating with my mighty PhD degree, letters that would presumably add all sorts of things to my name that people tend to butcher to pronounce simply because they are trying to too hard. I doubt having the letters behind my name will help people to realize that my name reads just as it writes, nothing fancy.
Anyways, do I think I am going to defend my thesis in three months, I do not think so, although I do not have any choice not to defend, since I am already starting another program in the fall. But my lab book and cells seem to be telling me if you are only lucky, you will make it to a medical school with your sanity and hopefully a degree behind your name, but only if "we," referring to my lab book and cells, decide you are fit to be let go.
See what I mean, I am talking to myself and referring my lab book and cells like they are my defense committee, which in reality they really are. If they are good enough, you will graduate, if they are not, you will not. Right at this moment, I am not graduating... Yes getting out is the hardest part of getting a PhD.