Let’s say a patient goes into the hospital to have part of his right leg amputated, but, due to an error, the doctor amputates the wrong leg. That appears to be a prime case of negligence. Other cases are more difficult to prove, as when a physician fails to diagnose a form of cancer. The case is more difficult because it is far less obvious that the physician failed to adhere to a reasonable standard of care for physicians. Many professional negligence claims require the plaintiff to produce expert testimony showing that the defendant professional violated the applicable standard of care.