Pathognomonic

Pathognomonic: A sign or symptom that is so characteristic of a disease that it makes the diagnosis. For example, Koplik's spots (on the buccal mucosa opposite the lst and 2nd upper molars) are pathognomonic of measles. The word "pathognomonic" (pronounced patho-no-monic) comes from the Greek "pathognomonikos' meaning "skilled in judging diseases."