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Rash is a nonspecific term that may refer to many acquired skin conditions. As a visual speciality, dermatology comprises several categories of skin disease that are based primarily on clinical morphology and pathophysiology. Dermatologic conditions that are of primary interest from an infectious diseases perspective include bacterial and mycobacterial infections, fungal infections, protozoal infections, viral infections, reactive erythemas, drug- and virus-induced hypersensitivity syndromes, and vasculitic diseases and purpura (Table 17-1).

TABLE 17-1Diagnostic Classification of Selected Infectious and Non-Infections Etiologies of Rashes

Establishing the differential diagnosis of a rash relies on an appropriately focused history and on correctly describing the morphology and distribution of the rash. Without a correct assessment of these features, it is difficult to arrive at an appropriate differential diagnosis. A brief overview of common terms used in dermatology is presented in Table 17-2. Common patterns of distribution are presented in Table 17-3.

TABLE 17-2Common Morphologic Patterns of Dermatologic Disease

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