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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Ovoid/popcorn-like soft-tissue calcifications suggest systemic sclerosis, but also occur in dermatomyositis, polymyositis/overlap disease, tumor calcinosis, tenosynovitis with psammomatous calcifications, and some cartilaginous tumors.

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Wickham striae mechanism. T-cell–mediated interface dermatitis. CD8+ T cells destroy basal keratinocytes, causing degeneration of the stratum basale and a lymphocytic infiltrate at the dermal-epidermal junction. Hypergranulosis and orthokeratosis follow, producing the striae.

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History of the pathognomonic finding in Aicardi syndrome: chorioretinal lacunae. They were thought to be scars left over from chorioretinitis, but Hoyt and colleagues’ 1978 paper showed it was a noninflammatory process in the retinal pigment epithelium and choroid.

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Sign spotlight: Gottron’s papules. Small, raised, flat-topped, pink-red to violaceous, scaly bumps that classically sit over the knuckles and other extensor joints of the hands. Think Dermatomyositis.

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Are Koplik spots pathognomonic for measles? Probably not. In a 2019 series of 717 patients labeled as having Koplik spots, PCR found measles in 202, rubella in 125, several other viruses, and no tested virus in 339. The major limitation: no photographs for validation.

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Here's strawberry gingivitis. In granulomatosis with polyangiitis, the threshold for ANCA-mediated autoimmune activity lowers, and that means tons of antigenic stimulation in places like the nose and mouth, leading to granulomatous inflammation.

#pathognomonic #meded