Click for a larger image
UPDATED: This is a medical word that originates from Greek roots. The word [pathos] meaning "disease", and the word [γνωμο] (gnomos) or [gnomonikos] meaning "opinion", "judge" or "someone fit to judge". The word [pathognomonic] refers to a symptom, a sign, or a combination that, by its presence, defines or diagnoses a specific pathology or condition.
Most pathognomonic diagnoses are attained by a combination of symptoms and signs. Single pathognomonic events are not common. An example would be the presence of Koplik's spots in the buccal mucosa close to the exit of the parotid duct which are pathognomonic for measles. Koplik's spots (named after Dr. Henry Koplik) are small white spots with a reddish background. In addition to this location, Koplik's spots can be found occasionally on the conjunctiva, vaginal mucosa, or gastrointestinal mucosa1.
Sources:
1. Steichen, O., & Dautheville, S. (2009). Koplik spots in early measles. Can Med Assoc J 180 (5), 583
2. Koplik H. The diagnosis of the invasion of measles from a study of the exanthema as it appears on the buccal mucous membrane. Arch Pediatr 1896;13:918-22.
3. "Despedir al Sarampion" Article in Spanish by Dr. Luis Vasta.