Actors are hired. Each is assigned an illness or condition and a set of symptoms to match. The actors then tell medical students their symptoms.
Actor-patient Jim Malloy was told to portray the symptoms of an abdominal aortic aneurysm, which is when the main blood vessel that brings blood to the abdomen, pelvis and legs becomes enlarged. The condition can grow for many years without symptoms. It is most commonly seen in men over the age of 60 who have emphysema, genetic risks, high blood pressure, high cholesterol, obesity and who were or currently are smokers.
Left untreated, if the aneurysm expands too quickly it can burst open or leak blood along the blood vessel walls. This could lead to internal bleeding and death. Fast-growing aneurisms that are larger than two inches across are often candidates for surgical removal.
When Jones examined Malloy, he actually found an abdominal aortic aneurism. The student initially thought Malloy may have been a decoy, but thought it was best to mention that he detected something real.
"He thought I might have been a ringer that was planted in there to test him, and I had no symptoms,” Malloy explained. “He thought I was a plant with the real situation."
Malloy himself didn’t know there was anything wrong with him.
http://www.cbsnews.com/news/uva-medical-student-diagnoses-actor-with-life-threatening-condition-during-practice-exam/