Pathognomonic

Pathognomonic (pathognomonic), or pathognostic (from Greek pathos- suffering and gignosco- I know), a term used primarily in clinical medicine to denote a sign or symptom that is absolutely characteristic and distinctive for a particular disease (rarely used synonym: "diacritical" sign). One can distinguish between positive pathognomonic signs, which prove the presence of a particular disease, and negative ones, which exclude it (e.g., a dense pinpoint red rash is positively pathognostic for scarlet fever, but negatively so for measles). In modern medicine, the term pathognomonic is used significantly less frequently than before, since the vast majority of clinical symptoms, as gradually became clear, turned out to be only relatively characteristic of one disease or another. Therefore, diagnosis is usually based not on a single "pathognostic" symptom, but on the entire collection of signs and the nature of the disease's course.