"... DXplain, a decision support system developed at the Laboratory of Computer Science at the Massachusetts General Hospital, has the characteristics of both an electronic medical textbook and a medical reference system. In its reference or case analysis mode, DXplain accepts a set of clinical findings (signs, symptoms, laboratory data) to produce a ranked list of diagnoses which might explain (or be associated with) the clinical manifestations. DXplain provides justification for why each of these diseases might be considered, suggests what further clinical information would be useful to collect for each disease, and lists what clinical manifestations, if any, would be unusual or atypical for each of the specific diseases.
In the role of a medical textbook, DXplain can provide a description of over 2200 different diseases, emphasizing the signs and symptoms that occur in each disease, the etiology, the pathology, and the prognosis. DXplain also provides up to 10 references for each disease, selected to emphasize clinical reviews where these were available. In addition, DXplain can provide a list of diseases which should be considered for any one of over 4900 different clinical manifestations (signs, symptoms, and laboratory examinations)..." (from the link above)