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

image What is the differential diagnosis of a rash? How would you frame the differential?


In clinical practice, rashes are diagnosed through pattern recognition probably more than 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 recognized, and physicians often reach premature closure on an incorrect diagnosis.

The differential diagnosis of a rash should be based 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. This process can be complicated. Identifying the primary lesion may be difficult as it is often affected by secondary changes such as excoriation, erosion, crusting, or coalescence. The differential diagnosis of 1 lesion can also be extensive. Once the morphology of the primary lesion is identified, the next step in making the diagnosis is often to observe the distribution of lesions. 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.

  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, 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

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. Figure 29-1 presents an algorithm of a possible approach to patients with rashes and skin lesions.

  1. Papulosquamous eruptions (papules and plaques)

    1. Eczematous dermatitis

      1. Atopic dermatitis

      2. Allergic contact dermatitis

      3. Irritant contact dermatitis

    2. Pityriasis rosea

    3. Tinea infections

    4. Psoriasis

    5. Seborrheic dermatitis

  2. Folliculopapular eruptions (perifollicular papules)

    1. Acne vulgaris

    2. Rosacea

    3. Folliculitis

    4. Perioral dermatitis

  3. Dermal reaction patterns

    1. Urticaria

    2. Sarcoidosis

    3. Granuloma annulare

    4. Erythema nodosum

  4. Purpura and petechiae

    1. Palpable purpura

      1. Leukocytoclastic vasculitis

        • (1) Henoch-Schönlein purpura

        • (2) Allergic vasculitis

      2. Infectious

        • (1) Bacteremia

        • (2) Rocky Mountain spotted ...

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