• Clinical science

Principles of dermatology


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 or papules) appear as a direct result of a disease process. Secondary lesions such as scales or 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 may require laboratory testing or biopsy to confirm the diagnosis. Dermatological diseases are managed with medication (topical and systemic) and procedures such as surgery, cryotherapy, radiotherapy, or phototherapy. Topical treatments are often the first choice because they cause fewer systemic side effects and are easily administered.

Physical examination

  • Physical examination
  • Definition of lesions (primary/secondary lesions)
  • Description of lesions
    • Number (single/multiple)
    • Size
    • Color: e.g., pinkish discoloration in cutis marmorata
    • Texture: e.g., atrophic, calloused, crusty, verrucous (a cutaneous lesion that mimics lesions of verruca vulgaris)
    • Shape: e.g., round, oval, annular
    • Distribution
      • Symmetric/asymmetric
      • Unilateral/bilateral
      • Diffuse/grouped
  • Basic examination techniques
    • Palpation: evaluation of consistency (e.g., softness, firmness) and depth
    • Typical signs
    • Dermatoscope inspection
  • Diagnostic methods
    • Biopsy (histological examination of lesions)
    • Laboratory studies

The mouth, scalp, and nails should not be overlooked!


Primary lesions

Primary Lesion Description


  • A flat (nonpalpable) skin lesion ≤ 1 cm in size that differs in color from surrounding skin
Patch (dermatology)
  • A flat skin lesion > 1 cm in size that differs in color from surrounding skin
  • A small, palpable skin lesion ≤ 1 cm in diameter
Nodule (dermatology)
  • An elevated lesion, > 1 cm in both diameter and depth
Plaque (dermatology)
  • Palpable, usually raised lesion > 1 cm
Vesicle (dermatology)
  • Small fluid-containing blister (collection of fluid in the skin)≤ 1 cm in diameter
  • Large fluid-containing blister > 1 cm in diameter
Urticaria (hives)
  • Sharply demarcated lesions on edematous skin
  • Irregular borders
  • Transient (hours to days)


Secondary lesions

Secondary Lesions Description
Scale (dermatology)
  • Thickened stratum corneum
  • Scales are dry and usually whitish.
  • In contrast, crust is often moist and yellowish or brown.
  • Liquid debris (e.g., dried pus or blood)
Fissure (crack, 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 sometimes reaching the outer layer of the dermis)
  • Dead skin tissue
Skin atrophy
  • Composed of new connective tissue that has replaced lost substance
  • An overgrowth of scar tissue manifests as keloid (thickened, raised tissue that grows beyond the borders of the scar and shows no regression).


Complex lesions

Lesions Description



  • Caused by bleeding into subcutaneous tissue, muscle, organ tissue or a cavity
    • Immediately after trauma: red
    • After 24–96 h: dark red; purple; blue/black
    • After 4–7 days: dark green
    • After 7 days: yellow; brownish
Subtypes of hematomas = purpura Nonpalpable purpura


  • 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)
  • Rash confined to the mucous membranes
  • 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



Histopathologic finding Characteristics Examples

Dermatologic therapy

Treatment options

The external nature of the skin allows a variety of treatment options, including:

  • Systemic medications
  • Topical medications
  • Physical procedures
    • Surgery
    • Cryotherapy (treatment that uses liquid nitrogen to expose abnormal tissue to very low temperatures and destroy malignant or premalignant cells; often used to treat skin lesions)
    • Radiotherapy
    • Phototherapy

Types of topical preparations

Drugs must be absorbed into the skin to be effective. Therefore, choosing the proper type of topical preparation for the pharmacological agent is important. Examples include:

  • Creams
  • Ointments
  • Lotions, foams, and gels

Topical steroids

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

  • Advantages
    • High therapeutic value
    • Relatively safe: few local and systemic adverse effects
  • Most common side effects:
  • Common examples:
Agent Potency Indication
Hydrocortisone (1%) Low For mild and chronic dermatoses and for use on the face
Triamcinolone (0.1%) Medium For intermediate severity
Clobetasol (0.05%) High For more severe dermatoses