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"Listen to the patient, he is telling you the diagnosis." This quotation, attributed to Sir William Osler, emphasizes what every medical student learns in medical school: history is more than 80 percent of diagnosis. To this, I would like to add, "Look at the skin, it reveals the diagnosis."

At its core, the Guidebook addresses how to diagnose in dermatology. Cognitive psychologists have shown that diagnostic reasoning takes place through activation of "scripts." Scripts describe the memory structure of clinical medical knowledge. They are networks of relevant knowledge and experience that direct selection and interpre-tation of new information. Expert dermatologists have, through experience, amassed a repository of scripts and have encountered a myriad of patterns on the skin. Imagine a patient presenting with an itchy linear rash after gardening. Instantly, knowledge of acute contact dermatitis pops into the clinician's mind. Accompanying this knowledge is a script of physical examination findings that confirm diagnosis and a script of treatment options, both based on the clinician's prior experience. Next, a patient presents with a longstanding rash over the elbows and knees, and joint pain. The prior script leaves active memory and is replaced by knowledge of psoriasis. Pattern recognition is another powerful processing strategy employed in dermatology. Seasoned clinicians have a repository of visual scripts that, in the absence of history, can direct diagnoses.

In the examples above, seeing linear erythematous plaques with vesicles, or well-demarcated plaques with silvery scale on the elbows and knees, with no history at all, will instantaneously call out acute contact dermatitis and psoriasis to the experienced clinician. For the novice, no such internal pathways exist. The novice needs to acquire knowledge of presentations and disease, as well as a set of rules to determine actions based on this knowledge. Even experts have not encountered the entirety of clinical presentations and can be stumped. When they encounter a novel or unusual eruption, they too need a way to stimulate further thought, a way to activate pre-existing scripts. The algorithms and pathways in this book are the tools or "rules" that can be used to guide one to a differential diagnosis, until such time as experience is accrued, and scripts and pattern recognition can take over.

There are two ways to create differential diagnostic lists. The first is to use an etiologic classification. Here, diseases are grouped according to whether they are inherited or acquired. If acquired, we subdivide them according to whether they are infectious, inflammatory, neoplastic, or drug-induced by nature. Other categories here include metabolic/depositional, vascular, or traumatic.

Another way to classify dermatoses, and the one heavily used in this book, is on morphologic grounds: we group diseases that look the same together. For example, all vesicobullous diseases, whether they may be caused by infection or an inflammatory process, such as autoimmunity, can be grouped together. We would then distinguish these diseases by clinical features, such as the depth of the bulla (informed by how many intact bullae versus erosions are seen, and whether the bullae are tense or flaccid) and the associated secondary lesions present (bullae and urticarial plaques, for example, connote bullous pemphigoid, whereas bullae, erosions, and vegetating plaques connote pemphigus vegetans). We would then look at the grouping of lesions (annular vesicles connote linear IgA disease, whereas herpetiform vesicles suggest herpes simplex, herpes zoster, dermatitis herpetiformis, or pemphigus herpetiformis); their distribution (dermatitis herpetiformis likes extensor aspects of extremities, a variant of bullous pemphigoid resides on the palms and soles, and pemphigus foliaceus can be found in the seborrheic areas); and how the scalp, hair, nails, mucous membranes, and lymph nodes are affected (oral involvement is always seen in pemphigus vulgaris, but it is less frequent in bullous pemphigoid; mucous membrane pemphigoid may cause scarring alopecia on the scalp; and there may be enlarged lymph nodes palpable in paraneoplastic conditions, such as paraneoplastic pemphigus). This morphologic classification requires an intimate knowledge of dermatologic diseases and the way they present. It also requires a framework. The elements of wheel of diagnosis (outlined above), the reaction patterns, and the differential diagnostic lists outlined in this Guidebook provide this framework.

To start off, in Chapter 1, the concept of the wheel of diagnosis is introduced. We then delve into ways to think about the primary lesions in Chapter 2 and discuss the diagnostic clues that secondary lesions can offer in Chapter 3. Color is a powerful diagnostic determinant, explored in depth in Chapter 4. In Chapter 5, reaction patterns are outlined and summarized. A reaction pattern refers to the lesions the skin produces in disease and comprises those dermatoses that have the same characteristic morphologic findings. For example, the papulosquamous reaction pattern connotes those dermatoses that manifest with scaly papules and plaques. In Chapters 6 through 10, each of the reaction patterns is reviewed at length, bringing in elements of the diagnostic wheel. The wheel of diagnosis, the reaction patterns, and color differentials may be used in sum or in part to create a differential diagnosis. By Chapters 11 and 12, we have moved on to discussing the grouping and configuration, and the distribution of lesions, respectively. The scalp, hair, mucous membranes, and nails yield important diagnostic clues, and examination of these areas is vital. Chapter 13 is devoted to discussing these clues. Bedside diagnostic maneuvers are another tool in the diagnostician's toolbox. The scrape of a Q-tip or a wipe of mineral oil can bring out features of diagnostic significance, for example, and we explore these in the last chapter, Chapter 14.

Whether you are a medical student or a dermatology resident encountering the field of dermatologic diagnosis for the first time, an internist or family practitioner looking to develop your approach to dermatology, or a seasoned clinician-dermatologist looking to augment your methodology of teaching differential diagnosis, this book is intended to be your guide, your authority, and your companion.

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