Introduction to Clinical Dermatology

“Thinking” Like a Dermatologist

The skin represents the largest organ of the human body. It consists of the epithelium, dermis, subcutaneous fat, and adnexal structures (hair follicles and glands), as well as supportive structures (blood vessels and nerves), all of which function to protect and maintain homeostasis.

Dermatology is a field of medicine that focuses on the skin, adjacent mucosa (oral and genital), and other adnexal structures (e.g., hair, nails, and sweat glands). Undoubtedly, dermatologists are the most adroit diagnosticians with regard to skin disease. However, much of their acumen comes from pattern recognition, a cultivated appreciation for diagnostic subtleties, and trained recognition of historical factors that make one particular disease more likely than another.

Thus, any clinician may improve when diagnosing and treating skin ailments simply by learning to think like a dermatologist. This includes fostering an appreciation for the classification schemes used in dermatology and for learning descriptive terminologies used by dermatologists.

Etiologic Premises

With regard to etiology, one of the most basic branch points in dermatology is to decide if a skin condition is neoplastic (benign or malignant) or inflammatory (e.g., rash, infection, autoimmune condition). Although it is likely that inflammatory conditions will prevail in urgent care and emergency settings, one must realize that a patient may present to these types of clinics with a neoplasm that has been ignored too long, until it can no longer be neglected.

Moreover, on occasion, there is visual and conceptual overlap with regard to inflammatory versus neoplastic conditions. For example, mycosis fungoides, the most common form of cutaneous T cell lymphoma, is a clonal lymphoproliferative disorder (a neoplasm), yet its clinical presentation resembles that of an inflammatory disorder. Conversely, sarcoidosis is an inflammatory condition that may present with discrete nodular lesions that mimic those of a neoplasm.


In dermatology, the term morphology refers to the appearance of a skin lesion(s), irrespective of the underlying pathophysiology. For example, a small blister is referred to as a vesicle, whether it is due to an infection (e.g., herpes simplex) or autoimmune condition (e.g., bullous pemphigoid).

Therefore, it is important to use correct morphologic terms to classify skin diseases. This is because these terms represent a native language, or lexicon, that allows professionals to describe skin disease in a consistent manner.

There are primary morphologic terms ( Table 1.1 ), which refer to the characteristic appearance of skin lesions (e.g., papule ), and secondary morphologic terms ( Table 1.2 ), which are used in addition to primary morphologic terms. These secondary morphologic terms reflect exogenous factors or temporal changes that evolve during the course of a skin disease.


Primary Morphologic Terms

Morphologic Term Salient Features Classic Disease Image Classic Diagnoses
Macule (or patch)

  • Flat lesion

    • <1 cm in diameter (macule)

    • >1 cm in diameter (patch)

  • Circumscribed

  • Color change that cannot be appreciated by tactile sensation alone

  • Vitiligo (photo)

  • Café-au-lait spot

  • Flat component of exanthems (measles)

  • Freckle

  • Lentigo


  • Elevated lesion

  • Usually <1 cm in diameter

  • Often with other secondary features (e.g., scale, crust)

  • Lichen nitidus (photo)

  • Elevated component of exanthems (measles)

  • Melanocytic nevi

  • Verruca or molluscum

  • Lichen planus

  • Guttate psoriasis


  • Elevated lesion

  • Usually >1 cm in diameter

  • Nonvesicular

  • Often with other secondary features (e.g., scale, crust)

  • Psoriasis vulgaris (photo)

  • Lichen simplex chronicus

  • Eczematous plaques

  • Granuloma annulare

  • Sarcoidosis


  • Large elevated lesion

  • Usually ≥2 cm in diameter

  • Involves the dermis and may extend into the subcutis

  • Neurofibromata (photo)

  • Basal cell carcinoma

  • Cutaneous lymphoma

  • Erythema nodosum

  • Lipoma


  • Small elevated lesion

  • <1 cm in diameter

  • Filled with clear fluid

  • Circumscribed

  • Herpes simplex infection (photo)

  • Varicella zoster infection

  • Dermatitis herpetiformis


  • Elevated lesion

  • Usually >1 cm in diameter

  • Filled with clear fluid

  • Circumscribed

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