Are Reinke crystals pathognomonic for Leydig cell tumor? Not really. They DO indicate a Leydig cell, but being a Leydig cell alone is not a crime...or a disease. So calling it "pathognomonic" is not correct, nor does it mean the cells are malignant.

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Christmas tree rash mechanism? Since the cause of pityriasis rosea is uncertain, this mechanism is also uncertain. The trunk lesions seem to align Langer's lines.

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From the pages of Pathognomonic Signs of Disease: American dermatologist Frank Crowe published his findings in 1964 where approximately 20% of NF1 cases he saw had axillary freckling. It was dubbed "Crowe's sign" in his honor.

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From the pages of Pathognomonic Signs of Disease: Giant cytoplasmic granules in leukocytes. These are oversized azurophilic inclusions on Wright-Giemsa smear, especially in neutrophils. This is a historically pathognomonic finding for Chédiak-Higashi syndrome.

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Are Call–Exner bodies pathognomonic for granulosa cell tumor? Not strictly. “Call–Exner-like” spaces may contain mucin or other eosinophilic material instead of basal lamina material as in true Call-Exner bodies.

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An Inflamed DIP joint means swelling plus redness at the distal interphalangeal joint (not just bony enlargement). It's an important clue to psoriatic arthritis, but also happens in gout, pseudogout, and erosive osteoarthritis.

This and others at pathognomonic.com

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Choroid tubercle mechanism: Hematogenous tuberculosis seeds the choroidal vasculature, where granulomatous inflammation produces pale yellow choroidal lesions. The choroid is so susceptible in part because it has the highest blood flow per unit tissue in the body.

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Dutch ophthalmologist Jan van der Hoeve coined the term "phakoma" in 1923 after the Greek phakos for ""lentil,"" as he observed multiple "mother spots" and round birthmarks on patients with tuberous sclerosis. Retinal phakoma are considered pathognomonic for the disease.

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Lisch Nodules: Small, dome-shaped, well-defined tan to brown raised spots on the surface of the iris, typically multiple, smooth, and visible on slit-lamp eye exam rather than to the naked eye. The academy tells us this automatically points to neurofibromatosis type 1.

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Is erythema migrans pathognomonic for Lyme disease?

Well, no.

STARI can produce an indistinguishable expanding rash, and similar lesions occur with Asian tick-associated rash illness; tick saliva may sometimes cause the eruption without Borrelia.

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PAS-positive macrophages point to Whipple’s disease. Key mimic: MAC, which is PAS-positive but acid-fast positive; T. whipplei is acid-fast negative. Rhodococcus and fungi can also complicate specificity. That's why this is not truly pathognomonic.

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Burton’s line mechanism: Lead in gingival capillaries reacts with hydrogen sulfide made by oral anaerobic bacteria, depositing dark lead sulfide at the gumline. It's an elegant intersection chemistry, toxicology, and microbiology.

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The moment anterior lenticonus enters the pathognomonic arena: A paper published in 1966 by Arnott, Crawfurd, and Toghill details a case report and in-depth literature review linking anterior lenticonus as a specific sign of Alport’s syndrome.

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Pneumatosis intestinalis: linear or bubbly gas within the bowel wall on abdominal imaging. When this pattern appears in preterm neonates, necrotizing enterocolitis should move high on the differential.

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Is electrical alternans pathognomonic for a large pericardial effusion? Not even close. It also occurs with rapid supraventricular rhythms, myocardial ischemia, intermittent bundle or fascicular block, hypertrophic cardiomyopathy, severe hypocalcemia, and drug toxicity.

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Howell–Jolly bodies are nuclear remnants inside RBCs, seen during hyposplenia. A worthwhile association to know, but far from pathognomonic. They also show up in certain chemotherapies and corticosteroids, megaloblastic anemia, myelodysplastic syndrome, and hemolytic anemia.

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Bilateral conjunctival suffusion, the hallmark of leptospirosis...on exams. Its mechanism: Leptospira injures conjunctival endothelium and loosens junctions, causing vascular leak and red eyes without purulent conjunctivitis.

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Entamoeba histolytica has infected humans since prehistoric times. The earliest confirmed detections of E. histolytica come from Neolithic sites in Switzerland dating to approximately 3700 to 3400 B.C.E., with additional evidence from ancient Greece and Bronze Age France.

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Maltese crosses in RBCs: four small pear-shaped intraerythrocytic forms radiate from a central point on Giemsa/Wright stain. Pathognomonic for babesiosis? Almost, but there's an edge case that violates strict pathognomony. Its name is Theileria.

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Are Lines of Zahn pathognomonic for antemortem thrombosis? Yes—when defined as laminated platelet-fibrin bands formed in flowing blood. Other fibrin patterns may occur after death, but they are not true Lines of Zahn; no postmortem process reproduces the genuine pattern.

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