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People only see what they are prepared to see.

—Ralph Waldo Emerson

Whether you are viewing Figure 1-1 in a book, in an aquarium, or in the sea, you immediately recognize the image as a fish. Those of you who are more schooled in the classification of fish might recognize that this is an angelfish with the tail resembling the head of the angel and the posterior fins representing the wings. If you are truly prepared to see this fish in all its splendor, you would see the blue circle above its eye as the crown of the queen angelfish.

Figure 1-1

Queen angelfish (Holacanthus ciliaris). (Courtesy of Sam Thekkethil. http://www.flickr.com/photos/natureloving.)

Making a diagnosis in medicine often involves the kind of pattern recognition needed to identify a queen angelfish. This is much the same as recognizing a beautiful bird or the painting of a favorite artist. If you are prepared to look for the clues that lead to the identification (diagnosis), you will see what needs to be seen. How can we be best prepared to see these clues? There is nothing more valuable than seeing an image or a patient who has the condition in question at least once before you encounter it on your own. The memory of a powerful visual image can become hard wired into your brain for ready recall.

In medicine it also helps to know where and how to look to find the clues you may need when the diagnosis cannot be made at a single glance. For example, a patient with inverse psoriasis may present with a rash under the breast that has been repeatedly and unsuccessfully treated with antifungal agents for candidiasis or tinea (Figures 1-2 and 1-3). The prepared clinician knows that not all erythematous plaques under the breast are fungal and looks for clues such as nail changes (Figures 1-2 and 1-4) or scaling erythematous plaques around the elbows, knees, or umbilicus (Figure 1-3). Knowing where to look and what to look for is how an experienced clinician makes the diagnosis of psoriasis.

Figure 1-2

Inverse psoriasis under the breast that might appear to be a fungal infection to the untrained eye. Note the splinter hemorrhages in the nail of the third digit that provide a clue that the patient has psoriasis. (Courtesy of Richard P. Usatine, MD.)

Figure 1-3

The patient in Figure 1-2 with inverse psoriasis had a typical psoriatic plaque in the umbilicus. This was the only other area involved besides the breasts and the nails but easily could have been missed without the ...

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