"What else could it be?" These are five powerful words that you should be ready to ask your doctor during a visit. If you are like most people you probably aren't looking forward to a visit to see the doctor. As a patient you don't have the luxury of always knowing exactly why you are seeing a doctor. All you know is that you have a handful of symptoms which are currently adding up to a decrease in your quality of life. Unfortunately your doctor doesn't know why you are seeing him either. He must play the role of detective and use all the clues he can find to discover what is ailing you. So here you are in the exam room as your doctor asks you a bunch of questions and examines you to validate your symptoms and discover any others that might present themselves. During this examination it is your doctor's job to piece together the information gathered and come up with a diagnosis. This is done through a process called Differential Diagnosis.
Your doctor's process should be similar to the Sherlock Holmes adage "When you have eliminated the impossible, whatever remains, however improbable, must be the truth." Using clues provided by your descriptions of the symptoms, data from any medical tests you have had, and his medical knowledge your doctor makes a list of all the possibilities. Then using the same information used to make the list he dismisses the items by ruling out those that don't fit the clues, until he is left with one diagnosis that fits all of the data.