I don't see her exiting the race of her own free will. It will take an Act of God (or an act of medical malpractice).
The general assumption is that VIPs all get the best medical care. That's not necessarily the case. The health care that VIPs get is the health care that VIPs want, and it's often the worst possible.
=> Clinton's physician's letter (scroll down)
As Dr. Drew has pointed out, there are some questions about the quality of care that she is getting, just from the letter.
She will be able to fake whatever is needed for a medical discharge from the race.