The petechial rash of Rocky Mountain spotted fever and other spotted fever group rickettsioses results primarily from which pathogenetic event?

• A Direct cytopathic destruction of keratinocytes by the organism
• B Platelet sequestration secondary to disseminated intravascular coagulation
• C Immune complex deposition in dermal capillaries
• D Infection of vascular endothelium causing vasculitis and increased permeability ✓

Explanation

Rickettsiae are obligate intracellular organisms that invade and multiply within vascular endothelial cells, inducing endothelial injury, perivascular lymphocytic infiltrates, vasculitis and microinfarcts. The resulting increased vascular permeability produces the rash, oedema, hypovolaemia and in severe cases shock and non-cardiogenic pulmonary oedema. DIC is not the primary mechanism, though thrombocytopaenia occurs. This endotheliotropism also explains meningoencephalitic and renal manifestations seen in severe rickettsial disease.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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