I ll give this 5 out 5 because this is one of the best executed film on such subject.
YES, it is hard to make a psychological thriller because with this type of movies one have to keep in mind the real psychological aspects of a patient and also the mix of fiction and art of story telling.
And till the last minute of this movie, no one can guess what is gonna happen and who is the real killer or it was an accident or it was a killing and what is the role of hospital.
You wanna know? Go and give it a try. You wont regret.