Principles of dermatology

Last updated: November 3, 2023

Summarytoggle arrow icon

Dermatology is the branch of medicine concerned with the skin, hair, and nails as well as the conditions associated with them. Basic knowledge of dermatology is essential for every physician, as approximately 50% of skin-related consultations are initially assessed by non-dermatologists. In the United States, the most common conditions seen by dermatologists include acne, actinic keratoses, non-melanoma skin cancers, benign tumors, and contact dermatitis. Skin lesions may be primary or secondary. Primary lesions (e.g., macules and papules) appear as a direct result of a disease process. Secondary lesions (e.g., scales and ulcers) may develop from primary lesions or result from external trauma (e.g., infections, scratching). Dermatological conditions can often be diagnosed based on patient history and physical examination but confirming the diagnosis may require laboratory testing or biopsy. Dermatological diseases are managed with medication (topical and systemic) and procedures such as surgery, cryoablation, radiotherapy, or phototherapy. Conservative treatment with topical agents is the treatment of choice for most dermatological conditions, with systemic medication and/or surgery being employed only if necessary. The prevalence, manifestations, and treatment of dermatological conditions in skin of color may vary depending on skin tone and genetic inheritance.

Patient historytoggle arrow icon

Physical examinationtoggle arrow icon


  • Complete skin assessment: Examine the skin (including the hands, mouth, and scalp) and nails for signs of dermatological conditions.
  • Examination techniques
  • Confirmation of diagnosis: may require biopsy for histopathological examination (see “Dermapathology” below) and/or laboratory studies

The hands, mouth, scalp, and nails should not be overlooked during a dermatological examination.

Skin examination

  • Determine the type of lesion: See primary skin lesions, secondary skin lesions, and complex skin lesions below.
  • Record lesion characteristics
    • Location
    • Number (single/multiple)
    • Size
    • Color: e.g., pink discoloration
    • Texture: e.g., atrophic, calloused, crusty, verrucous
    • Shape: e.g., round, oval, annular
    • Distribution
      • Symmetric/asymmetric
      • Unilateral/bilateral
      • Diffuse/grouped
    • Secondary changes (e.g., as a result of scratching)

Physical examination

Nail examination

Alterations of the nails
Clinical findings Clinical examples Possible underlying disease

Nail pitting


Nail clubbing


Mees lines


Examination of the hands

Primary skin lesionstoggle arrow icon

Overview of most common primary skin lesions [1][2]
Primary Lesions Description Cross-section Clinical example


Patch (dermatology)

Nodule (dermatology)
  • An elevated lesion, > 1 cm in both diameter and depth
Plaque (dermatology)
Vesicle (dermatology)

Secondary skin lesionstoggle arrow icon

Overview of most common secondary skin lesions [1][2]
Secondary Lesions Description Illustration Clinical example
Scale (dermatology)
Fissure (cleft)
Ulcer (dermatology)
  • Rounded or irregularly shaped deeper lesions that result from loss of the epidermis and some portion of the dermis.
Excoriation (scratch marks)
  • Abrasion produced by mechanical force, usually involving the epidermis (but may reach the outer layer of the dermis)
Skin atrophy
  • Thinning of skin without inflammation
  • Composed of new connective tissue that has replaced lost substance
  • An overgrowth of scar tissue manifests as a keloid (thickened, raised tissue that grows beyond the borders of the scar and shows no regression).

Complex skin lesionstoggle arrow icon

Overview of complex skin lesions [3]
Complex lesions Description Clinical example



  • Caused by bleeding into subcutaneous tissue, muscle, organ tissue or a cavity
    • Immediately after trauma: red
    • After 24–96 h: dark red, green, blue, purple, black
      • Cause: coagulation of the blood and degradation of hemoglobin into bile pigment
    • After 4–7 days: dark green
    • After 7 days: yellow; brownish
Purpura (a subtype of hematoma that does not blanch upon the application of pressure) Nonpalpable purpura


  • Flat, red-purple, pinpoint lesions < 3 mm in size

  • Flat, red-purple, larger form of petechiae, > 5 mm in size
Palpable purpura
  • Raised, red-purple lesions
Rashes Exanthem
  • Extended uniform rash (localized or generalized)
  • Reddening of the skin as a result of vasodilation (blanches if pressure is applied)
  • Generalized reddening of the skin
Maculopapular rash
Further lesions Lichenification
  • Thickening of the skin with accentuated skin markings

Differential diagnosis of annular skin lesionstoggle arrow icon

Overview of annular skin lesions
Etiology Location Characteristics
Erythema migrans
Tinea corporis
  • Face
  • Arms and legs
  • Trunk
Erythema marginatum
  • Trunk and extremities
  • Spares the face
  • Centrifugally expanding pink or light red rash with a well-defined outer border and a central clearing
  • Painless
  • Nonpruritic
Nummular eczema
  • Extremities
  • Well-demarcated coin-shaped lesions
  • Severely pruritic
  • Scabbing
Granuloma annulare
  • Localized form (more common): especially palms and soles
  • Generalized form (rare): involves trunk and extremities
  • Generalized involvement of the skin
Erythema multiforme (EM)
  • Symmetrical distribution
  • Backs of hands and feet first
  • May affect the entire body
  • EM minor: no involvement of mucous membranes
  • EM major: mucosal involvement (oral, ocular, genital ulcers)
Fixed drug eruption
  • Most commonly involves oral mucosa, trunk, hands, and/or genitals
  • Recurs in the same location on reexposure to the drug
Pityriasis rosea
  • Herald patch: typically on the trunk
  • Secondary eruption: trunk and extremities
Discoid lupus
  • Head
  • Face
  • Neck

Dermatopathologytoggle arrow icon

Histopathologic finding Characteristics Examples of associated conditions

Overview of treatmenttoggle arrow icon


  • Topical medications
    • First choice of treatment for most conditions; often preferred for treating dermatological conditions because they cause fewer systemic side effects
    • Examples: antibiotics, antifungals, corticosteroids, cytotoxic agents, antipruritics, retinoids
  • Systemic medications
  • Procedures
    • Surgery
    • Cryoablation: application of cold (e.g., using liquid nitrogen) to destroy abnormal tissue, malignant or premalignant skin lesions.
  • Other

Types of topical preparations [5]

  • Creams
  • Ointments
  • Lotions, foams, and gels

Topical steroids

Topical steroids are the most frequently used topical treatment in dermatology.

Agent Potency Indication
Hydrocortisone (1%) Low For mild and chronic dermatoses and for use on the face
Triamcinolone (0.1%) Medium For intermediate severity dermatoses
Clobetasol (0.05%) High For more severe dermatoses

Phototherapy [6][7][8]

Ultraviolet light is an effective treatment option for dermatological conditions because of its antiproliferative (e.g., slowing of keratinization) and antiinflammatory effects (e.g., inducing apoptosis of pathogenic T cells).

Skin of colortoggle arrow icon


  • Skin of color” refers to skin with a higher degree of pigmentation than that traditionally associated with Northern European heritage.
    • Skin tone is a spectrum that does not necessarily correspond to the skin tones traditionally associated with race.
    • Genetic inheritance can play a more important role in the manifestation of dermatological conditions than skin tone.
    • It is important to recognize with regard to the concept of “race” still in use by the US Census Bureau that there is no foundation for a scientific concept of race as a specific genetic, phenotypical, ancestral, or ethnic trait that distinguishes all members of one population from those of another.
  • Western medicine has traditionally focused on the presentation of light-skinned individuals, leading to underrepresentation in dermatological resources of cutaneous manifestations in individuals with skin of color.
  • Changing demographics and increased representation of historically marginalized populations have raised awareness for the need and importance of thoroughly understanding skin of color.


  • The incidence and prevalence of dermatological conditions in individuals with skin of color depend to a high degree on skin tone and differ from those seen in light-skinned individuals for a variety of reasons:
    • Underrecognition due to poor dermatological training in diagnosing cutaneous conditions in individuals with skin of color.
    • Factors that are influenced by different lifestyle and cultural practices
    • Genetic inheritance
Dermatological conditions that are more common in individuals with skin of color [13][14][15]
Heritage Condition(s) with higher prevalence
Southeast Asian
East Asian
West Asian
Latin American
American Indian