Dermatology MCQ - Inflammatory Dermatoses - Nummular dermatitis

A 55-year-old man with a history of very dry skin presents with several intensely pruritic, coin-shaped plaques on his lower legs and the dorsa of his hands. The lesions are well-circumscribed, erythematous, and exhibit fine scaling, tiny vesicles, and mild oozing. The rest of his skin is xerotic. Which of the following is the most characteristic clinical feature of this condition?

A. Annular configuration with a trailing scale
B. Symmetrical involvement of skin folds
C. Herald patch preceding the generalized eruption
D. Coin-shaped eczematous plaques
E. Telangiectasias and follicular plugging

Correct Answer: D. Coin-shaped eczematous plaques

Explanation:

• The presentation is classic for nummular dermatitis (also known as discoid eczema). The term "nummular" itself means coin-shaped, which is the hallmark of this condition. Key clues include the chronicity, location on distal extremities (legs are most common), intense pruritus, and the morphology of well-circumscribed, eczematous plaques that can be crusted, scaly, or vesicular.

• Why the other options are incorrect:

  • A. Annular configuration with a trailing scale: This describes the classic lesion of tinea corporis (a fungal infection) or erythema annulare centrifugum. Nummular dermatitis plaques are typically solid discs, not rings.

  • B. Symmetrical involvement of skin folds: This is the classic distribution of atopic dermatitis in adolescents and adults (antecubital and popliteal fossae). Nummular dermatitis favors extensor surfaces.

  • C. Herald patch preceding the generalized eruption: This is the pathognomonic initial sign of pityriasis rosea.

  • E. Telangiectasias and follicular plugging: These are features of discoid lupus erythematosus, a chronic scarring condition, not an eczematous one. Nummular dermatitis does not cause scarring or follicular plugging.

Key Points for Exams

• Epidemiology & Associations: It is more common in middle-aged and elderly males. It is strongly associated with xerosis (dry skin), especially in winter. Other associations include stasis dermatitis (hence the common leg involvement) and, less consistently, with atopy. It can be triggered or exacerbated by skin irritation and allergens.

• Differential Diagnosis: The main differentials are tinea corporis (a KOH preparation is essential to rule this out), psoriasis (usually thicker silvery scale), and allergic contact dermatitis (which may have a more irregular shape corresponding to allergen exposure).

• Prognosis: The course is often chronic and relapsing. Individual plaques can persist for weeks to months. Post-inflammatory hyperpigmentation is very common after the active inflammation resolves.

• Management:

  1. Emollients: Fundamental to repair and maintain the skin barrier and prevent xerosis-induced flares.

  2. Topical Corticosteroids: Medium to high-potency steroids are first-line to reduce inflammation and pruritus. Ointment-based formulations are often preferred for their superior moisturizing effect.

  3. Occlusion: Using wet wraps or hydrocolloid dressings over topical steroids can enhance penetration and healing, especially for thick, stubborn plaques.

  4. Treatment of Infection: Secondary bacterial infection (impetiginization) is common; signs of oozing, crusting, and pustules may warrant a course of systemic antibiotics.

  5. Severe/Refractory Cases: Phototherapy (e.g., NB-UVB) is a highly effective second-line treatment. Systemic immunosuppressants (e.g., methotrexate, cyclosporine) are rarely used for widespread, severe disease.