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Ms. N is a 23-year-old woman who comes to see you complaining of a rash.

Image not available.What is the differential diagnosis of a rash? How would you frame the differential?
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In clinical practice, rashes are diagnosed through pattern recognition probably more than with any other complaint. This is an effective way of making a diagnosis when the diagnosis is obvious or when the observer is very experienced. The risk with pattern recognition is that diagnostic hypotheses are heavily influenced by recent experience, rare diagnoses tend not to be considered, and physicians often reach premature closure on an incorrect diagnosis.

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The most useful way of organizing the differential diagnosis of a rash is to base it on the morphology of the lesion. To correctly categorize a lesion's morphology, the physician must first identify the primary lesion, the typical element of the eruption. Once the primary lesion is identified, the eruption can be categorized based on morphology and then the specific diagnosis identified. This process can be difficult. The primary lesion is often affected by secondary changes such as excoriation, erosion, crusting, and even coalescence. The differential diagnosis of one lesion can also be extensive. After determining the morphology of the primary lesion, the next step in making the diagnosis is often to observe the distribution of lesion. Some eruptions will have characteristic distributions. What follows are some important definitions, followed by a differential diagnosis of some of the most common primary lesions.

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  1. Macule: lesion without elevation or depression, < 1 cm

  2. Patch: lesion without elevation or depression, > 1 cm

  3. Papule: any solid, elevated “bump” < 1 cm

  4. Plaque: raised plateau-like lesion of variable size, no depth, often a confluence of papules

  5. Nodule: solid lesion with palpable elevation, 1–5 cm

  6. Tumor: solid growth, > 5 cm

  7. Cyst: encapsulated lesion, filled with soft material

  8. Vesicle: elevated, fluid-filled blister, < 1 cm

  9. Bulla: elevated, fluid-filled blister, > 1 cm

  10. Pustule: elevated, pus-filled blister, any size

  11. Wheal: inflamed papule or plaque formed by transient and superficial local edema

  12. Comedone: a plug of keratinous material and skin oils retained in a follicle; open is black, closed is white

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Papulosquamous eruptions present with papules and plaques associated with superficial scaling. Folliculopapular eruptions begin as papules arising in a perifollicular distribution. Dermal reaction patterns result from infiltrative and inflammatory processes involving the dermal and subcutaneous tissues. Petechia and purpura occur when there is leakage of blood products into surrounding tissues from inflamed or damaged blood vessels. Blistering disorders present with vesicles and bullae.

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  1. Papulosquamous eruptions (papules and plaques)

    1. Eczematous dermatitis

      1. Atopic dermatitis

      1. Allergic contact dermatitis

      1. Irritant contact dermatitis

    1. Pityriasis rosea

    1. Tinea infections

    1. Psoriasis

    1. Seborrheic dermatitis

  2. Folliculopapular eruptions (perifollicular papules)

    1. Acne vulgaris

    1. Rosacea

    1. Folliculitis

    1. Perioral dermatitis

  3. Dermal reaction patterns

    1. Urticaria

    1. Sarcoidosis

    1. Granuloma annulare

    1. Erythema nodosum

  4. Purpura and petechiae

    1. Palpable purpura

      1. Leukocytoclastic vasculitis

        1. Henoch-Schönlein purpura

        1. Allergic vasculitis

      1. Infectious

        1. Bacteremia

        1. Rocky Mountain spotted fever

    1. Nonpalpable purpura

      1. Thrombocytopenia

      1. Medication related

      1. Benign pigmented purpura

      1. Bacteremia

      1. Disseminated intravascular coagulation

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