Dermatology MCQ - Inflammatory Dermatoses - Mastocytosis

A 3-year-old child is brought to clinic for evaluation of multiple, 1-2 cm, reddish-brown macules and slightly raised papules scattered on the trunk. The mother reports that when the child cries or rubs the lesions, they become erythematous and swollen, and the child seems uncomfortable. The most pathognomonic clinical sign to confirm the diagnosis at the bedside is:

A. Auspitz sign
B. Koebner phenomenon
C. Darier's sign
D. Nikolsky sign
E. Herald patch

Correct Answer: C. Darier's sign

Answer & Explanation

Explanation:

The presentation of reddish-brown macules and papules that urticate (swell and become red) upon rubbing is classic for cutaneous mastocytosis, specifically urticaria pigmentosa in a child. Darier's sign is the pathognomonic clinical finding for this condition.

• Mechanism: Rubbing the lesion causes mechanical degranulation of the dense infiltrate of mast cells in the dermis, releasing histamine and other mediators. This leads to localized whealing, erythema, and often pruritus within minutes.

The other signs are associated with different dermatological diseases:

• A. Auspitz sign: The appearance of pinpoint bleeding after scraping off scale. This is characteristic of psoriasis.

• B. Koebner phenomenon: The development of new lesions at sites of skin trauma. This is seen in psoriasis, lichen planus, and vitiligo, but the reaction in mastocytosis is an immediate urticarial response of existing lesions, not the development of new ones.

• D. Nikolsky sign: The induction of epidermal separation with lateral pressure, seen in pemphigus vulgaris and staphylococcal scalded skin syndrome.

• E. Herald patch: A single, larger lesion that appears 1-2 weeks before the generalized eruption of pityriasis rosea.

Key Associations for Mastocytosis

• Pathophysiology: Mastocytosis is a heterogeneous group of disorders characterized by the abnormal accumulation and proliferation of clonal mast cells in one or more organs, most commonly the skin. The vast majority of cases are driven by a gain-of-function mutation in the KIT gene (most commonly D816V), which leads to uncontrolled mast cell growth and survival.

• Classification:

  • Cutaneous Mastocytosis (CM): Confined to the skin. This includes:

    • Maculopapular Cutaneous Mastocytosis (Urticaria Pigmentosa): The most common form in both children and adults.

    • Solitary Mastocytoma: A single, larger nodule or plaque.

    • Diffuse Cutaneous Mastocytosis: A rare, severe form with diffuse skin thickening and a high risk of systemic complications.

  • Systemic Mastocytosis (SM): Involves at least one extracutaneous organ (most commonly bone marrow) with or without skin involvement. This is more common in adults.

• Prognosis:

  • Pediatric-onset mastocytosis often has an excellent prognosis, with many cases resolving or significantly improving by puberty.

  • Adult-onset disease is usually persistent and is more often associated with systemic involvement.

• Management:

  • Patient Education: Avoid triggers of mast cell degranulation (e.g., temperature extremes, friction, alcohol, NSAIDs, opioids, certain anesthetics, iodinated contrast dye).

  • Symptomatic Control:

    • H1-antihistamines: For pruritus and flushing.

    • H2-antihistamines: For gastric acid-related symptoms.

    • Mast cell stabilizers: (e.g., Cromolyn sodium) for gastrointestinal symptoms and cutaneous symptoms.

  • Epinephrine Auto-injector: Must be prescribed for all patients with systemic disease or a history of anaphylaxis.

  • Treatment for Advanced Disease: For aggressive systemic mastocytosis, agents targeting the KIT mutation, such as midostaurin, or other cytoreductive therapies may be used.