Differential Diagnosis
Differential diagnosis is the process of comparing and contrasting the signs, symptoms, and laboratory findings of two or more diseases to determine which is most likely causing the patient's condition. The process is extremely valuable to students as they work through case studies because it provides a methodical approach to evaluate numerous pieces of information and, by process of elimination, arrive at a diagnosis. Building a list of differentials encourages students to consider multiple possiblities instead of jumping to a conclusion and then allows them to narrow down their list using the “evidence” or signs, symptoms and diagnostic test results that support various diagnoses. This can be a powerful tool to help teach critical thinking skills and deductive reasoning. It is best to encourage students to make their lists inclusive rather than exclusive, and then eliminate disorders from their initial list as they gain additional information.
A simple example - The differential diagnosis of rhinitis (a runny nose) includes allergic rhinitis (hay fever), nasal foreign body, and, of course, the common cold.
http://dictionary.webmd.com/terms/differential-diagnosis
http://www.medterms.com/script/main/art.asp?articlekey=2991