Pathognomonic in the context of "Medical diagnosis"

The Great Imitator (also the Great Masquerader) is a phrase used for medical conditions that feature nonspecific symptoms and may be confused with a number of other diseases. The term connotes especially difficult differential diagnosis, increased potential for misdiagnosis, and the protean nature of some diseases. Most great imitators are systemic in nature or have systemic sequelae, and an aspect of nonspecific symptoms is logically almost always involved. In some cases, an assumption that a particular sign or symptom, or a particular pattern of several thereof, is pathognomonic turns out to be false, as the reality is that it is only nearly so.

As recently as the 1950s, syphilis was widely considered by physicians to be "the great imitator", and in the next few decades after that, several other candidates, mainly tuberculosis but occasionally others, were asserted as being "the second great imitator". But because differential diagnosis is inherently subject to occasional difficulty and to false positives and false negatives, the idea that there are only one or two great imitators was more melodrama than objective description. In recent decades, more than a dozen diseases have been recognized in the medical literature as worthy of being considered great imitators, on the common theme of recurring misdiagnoses/missed diagnoses and protean manifestations. Nonetheless, not every caveat (not every mimic) meets the threshold, because it is inherent to differential diagnosis generally that there are thousands of caveats.

Myalgic encephalomyelitis​/​chronic fatigue syndrome (ME/CFS) is a disabling chronic illness. People with ME/CFS experience profound fatigue that does not go away with rest, as well as sleep issues and problems with memory or concentration. The hallmark symptom is post-exertional malaise (PEM), a worsening of the illness that can start immediately or hours to days after even minor physical or mental activity. This "crash" can last from hours or days to several months. Further common symptoms include dizziness or faintness when upright and pain.

The cause of the disease is unknown. ME/CFS often starts after an infection, such as mononucleosis and it can run in families. ME/CFS is associated with changes in the nervous and immune systems, as well as in energy production. Diagnosis is based on distinctive symptoms, and a differential diagnosis, because no diagnostic test such as a blood test or imaging is available.

Koplik's spots (also Koplik's sign) are a prodromic viral enanthem of measles manifesting two to three days before the measles rash itself. They are characterized as clustered, white lesions on the buccal mucosa (opposite the upper 1st & 2nd molars) and are pathognomonic for measles. The textbook description of Koplik spots is ulcerated mucosal lesions marked by necrosis, neutrophilic exudate, and neovascularization. They are described as appearing like "grains of salt on a reddish background", and often fade as the maculopapular rash develops. As well as their diagnostic significance, they are important in the control of outbreaks. Their appearance, in context of a diagnosed case, before they reach maximum infectivity, permits isolation of the contacts and greatly aids control of this highly infectious disease.

Nobel laureate John F. Enders and Thomas Peebles, who first isolated the measles virus, were careful to collect their samples from patients showing Koplik's spots.