What are the differential diagnoses for a patient with an unspecified skin condition, considering their medical history, allergies, current medications, age, sex, and ethnicity?

Differential Diagnosis for Unspecified Skin Condition

Without seeing the actual skin lesion, I cannot provide a definitive diagnosis, but I can guide you through a systematic approach to narrow the differential based on key clinical features you must identify.

Critical Initial Assessment

First, determine if this is a dermatologic emergency requiring immediate intervention:

• Look for multiple uniform "punched-out" erosions or vesiculopustular eruptions—this suggests eczema herpeticum, a true medical emergency requiring immediate systemic antivirals 1, 2
• Assess for signs of secondary bacterial infection: crusting, weeping, or honey-colored discharge 3, 1
• In immunocompromised patients, even localized lesions may represent systemic or life-threatening infection 3

Systematic Diagnostic Approach

Step 1: Characterize the Primary Morphology

Eczematous/Inflammatory Lesions:

• Atopic dermatitis requires an itchy skin condition PLUS three or more of: history of itchiness in skin creases, history of asthma/hay fever or atopic disease in first-degree relatives, general dry skin in past year, visible flexural eczema, and onset in first two years of life 3, 1
• In children under 4 years, atopic dermatitis characteristically affects cheeks or forehead rather than flexural areas 1
• Consider allergic contact dermatitis if there is unusual distribution, later onset, or disease not responding to standard therapy—patch testing should be performed 3
• Irritant contact dermatitis is more common than allergic contact dermatitis but cannot be distinguished by morphology alone 2

Erythrodermic Presentation:

• Differential includes atopic dermatitis, contact dermatitis, seborrheic dermatitis, cutaneous T-cell lymphoma, and pityriasis rubra pilaris 3
• Personal and family history of psoriasis, indurated plaques with silvery scale favor erythrodermic psoriasis over atopic dermatitis 3
• Obtain blood for flow cytometry and Sézary cell count to assess for cutaneous T-cell lymphoma 3

Nodular/Papular Lesions:

• Cutaneous Rosai-Dorfman-Destombes disease presents as slow-growing, painless, nonpruritic nodules, plaques, or papules with yellow to red to brown coloration 3
• Differential includes acne vulgaris, varicella-zoster virus, sarcoidosis, cutaneous lymphoma, and metastasis 3

Step 2: Identify Distribution Pattern

Facial/Cheek Involvement:

• In infants and young children, consider atopic dermatitis as primary diagnosis 1
• Assess for exposure to irritants like soaps, detergents, cosmetics, or topical medications 1

Palmar/Hand Involvement:

• Take detailed occupational history including materials, chemicals, or products handled at work 2
• Pattern and morphology alone cannot distinguish between atopic, irritant, and allergic etiologies 2
• Allergic contact dermatitis carries worse prognosis than irritant dermatitis unless allergen is identified and avoided 2

Flexural Distribution:

• Classic for atopic dermatitis in older children and adults 3, 1

Step 3: Obtain Tissue Diagnosis When Indicated

In immunocompromised patients, aggressively determine etiology by aspiration and/or biopsy of skin lesions and submit for cytological/histological assessment, microbial staining, and cultures 3

For interface dermatitis pattern on histology:

• Vacuolar interface dermatitis with perivascular lymphocytic infiltrate and mucin deposition suggests dermatomyositis (direct immunofluorescence negative) versus lupus (direct immunofluorescence positive) 4
• Lichenoid (band-like) lymphocytic infiltrate hugging dermoepidermal junction is hallmark of lichen planus 4

Common Pitfalls to Avoid

• Do not rely on morphology alone to distinguish atopic from contact dermatitis—patch testing is essential when contact allergy is suspected 3, 2
• Do not assume palmar eczema is purely endogenous/atopic without thoroughly investigating occupational and environmental exposures 2
• Do not delay treatment of suspected eczema herpeticum—this requires immediate systemic antivirals 1, 2
• In patients on BRAF/MEK inhibitors for melanoma, consider drug-induced cutaneous toxicities including rash, hyperkeratosis, keratoacanthoma, and cutaneous squamous cell carcinoma 3

Essential Diagnostic Workup

History must include:

• Detailed timeline of symptom onset and progression 3
• Personal and family history of atopy (asthma, hay fever, eczema) 3, 1
• Occupational and environmental exposures 3, 2
• Current medications and recent changes 3
• Presence and severity of pruritus and sleep disturbance 2

Physical examination must document:

• Extent and severity using validated tools 2
• Distribution pattern (localized vs. generalized, flexural vs. extensor) 3, 2
• Signs of secondary infection (crusting, weeping, pustules, uniform erosions) 3, 1, 2
• Presence of lymphadenopathy (usually secondary to extensive skin disease in otherwise healthy patients) 3

Laboratory/diagnostic studies when indicated:

• Bacterial culture swabs if infection suspected 3, 1
• Viral culture or electron microscopy if herpes simplex suspected 3
• Patch testing for suspected allergic contact dermatitis 3
• Skin biopsy for histopathology and immunofluorescence when diagnosis uncertain 3, 4