The Collaborative International Dictionary
Pathognomonic \Pa*thog`no*mon"ic\, a. [Gr. ? skilled in judging of diseases; ? a disease + ? skilled: cf. F. pathognomonique. See Gnomic.] (Med.) Specially or decisively characteristic of a disease; indicating with certainty a disease; as, a pathognomonic symptom.
The true pathognomonic sign of love jealousy.
--Arbuthnot.
Douglas Harper's Etymology Dictionary
Wiktionary
a. diagnostic beyond any doubt for a particular disease.
Wikipedia
Pathognomonic (often misspelled as pathognomic and sometimes as pathomnemonic) is a term, often used in medicine, that means characteristic for a particular disease. A pathognomonic sign is a particular sign whose presence means that a particular disease is present beyond any doubt. Labelling a sign or symptom "pathognomonic" represents a marked intensification of a "diagnostic" sign or symptom.
The word is an adjective of Greek origin (παθογνωμονικόν [σύμπτωμα]), derived from páthos ("πάθος" meaning "disease") and gnōmon ("γνώμον" meaning "judge").
Usage examples of "pathognomonic".
And since the only school of thought to which he was accustomed was essentially pathological and pathognomonic, he psyched himself up one side and down the other, growing more and more bewildered.
Well, perhaps the microscopic of the heart will show something pathognomonic, but I bet this case will fall into that category of an unknown coup de grace, at least specifically.