I was asked to answer this question on Quora, an informational web site.
Do doctors look up reference material when diagnosing a patient?
Actually, I learned everything in medical school. But there are some who did not learn everything, believe it or not.
Forgive my attempt at humor, but it is impossible to know everything in medicine. That is why there are specialties. Medicine is an ever evolving discipline, requiring constant study. Physicians are required to do a certain amount of CME (continuing medical education) every year.
There are always difficult cases, and doctors frequently consult with one another.
And if I was doing an unusual surgical case, I sometimes consulted a textbook or journal that I brought to the operating room with me just before going in for surgery.
Of course, routine cases are very simple, and our knowledge is immediate. One of the main things I learned early on in my training was to use "tincture of time". If it is not an emergency, you can always have the patient return for further evaluation at a later date.
All symptoms might not be present at the initial visit. Like the patient who goes to a doctor with a sore throat and is misdiagnosed. Then goes to a second doctor, but this time a rash is present. The doctor says, "You have measles." It was the same disease, just at a different stage. Is the first doctor an idiot, and the second a genius?