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The “wheel of diagnosis,” as depicted in Figure 1.1, encompasses the salient clinical features of skin lesions that can be used to make a diagnosis. The “spokes” or diagnostic boxes on this virtual wheel are the fundamental clinical features that render diagnostic information:

  • Identifying primary (and associated secondary) lesions

  • Grouping and configuration of these lesions

  • Studying the distribution of these lesions

  • Examining the lesions and gleaning information from the scalp, hair, nails, mucous membranes, and lymph nodes

  • Contextualizing these findings to the patient’s history

You may need to go through each diagnostic box on the wheel to make a diagnosis. Sometimes, however, one particular characteristic of a rash is so striking that this, by itself, may generate a differential diagnosis, such as grouping of primary lesions or their distribution. In this context, any of the other diagnostic boxes may be used to either confirm or narrow down a diagnosis. This chapter delves into each of the diagnostic boxes of the wheel in depth.


Primary lesions are those lesions that the body produces in any given eruption. They are the main determinants of diagnosis. In any widespread eruption, a vital first step is to seek out the primary lesions as this will determine the next diagnostic pathway that you choose. The primary lesions and their definitions are summarized as follows:

  • Macule (Figure 1.2A): A macule is a circumscribed, flat lesion. It is a flat area that differs from the surrounding skin only with respect to its color. Red, brown, purple, and white are the usual colors of macules, but exogenous pigments of almost any color may be found in tattoos, for example.

  • Plaque (Figure 1.2B): A plaque is a solid raised or depressed lesion that measures at least 1 cm in diameter. Raised plaques, as opposed to nodules, are broader than they are tall; that is, their surface area is greater than their height. Plaques may be scaly or smooth.

  • Patch: The term patch is used to connote any portion of skin that is different in appearance or character from its surroundings. In practice, dermatologists have used this term variably. Some restrict its use to mean large macules (those greater than 1 cm), and others use it to refer to relatively thin but large plaques. In general, accurate description can almost always be better served by employing the more precise terminology, such as a “large macule” or a “thin plaque.” You will see that this term is therefore not included in the wheel.

  • Papule (Figure 1.2C): A papule is a solid, circumscribed elevated or depressed lesion that measures less than 1 cm.

  • Nodule (Figure 1.2D): A nodule is similar in configuration to a papule, ...

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