What is the most likely diagnosis for a 1-year-old child presenting with a pruritic (itchy) rash on the scalp, face, trunk, and extensor surfaces?

Diagnosis: Atopic Dermatitis

The most likely diagnosis is atopic dermatitis (Option A), based on the characteristic age-specific distribution pattern of pruritic, red, scaly, and weepy lesions affecting the scalp, face, trunk, and extensor surfaces in a 1-year-old child. 1

Clinical Reasoning

Age-Specific Distribution Pattern

• In infancy (which includes 1-year-olds), atopic dermatitis classically starts on the cheeks and extends to the neck, trunk, and extensor surfaces of extremities 1
• The scalp and face involvement is typical for this age group 2, 3, 4
• Notably, atopic dermatitis spares the diaper area in infants, which helps distinguish it from other conditions 1, 5, 2

Characteristic Lesion Morphology

• The description of "red, scaly, and weepy" lesions is consistent with acute atopic dermatitis 1, 5
• Acute lesions present as erythema, exudation, papules, and vesiculopapules 1
• The weeping nature indicates acute phase disease with serous exudates 5, 2

Pruritus as Essential Feature

• Pruritus (itchiness) is a mandatory diagnostic criterion for atopic dermatitis at any age 1
• The presence of itching with the characteristic distribution pattern strongly supports this diagnosis 1

Why Not Seborrheic Dermatitis (Option D)

Seborrheic dermatitis can be differentiated from atopic dermatitis by several key features:

• Seborrheic dermatitis affects the groin and axillary regions, while atopic dermatitis spares these areas 1
• Seborrheic dermatitis tends NOT to be pruritic, whereas this child has significant itching 1
• Seborrheic dermatitis presents with greasy, yellow scales rather than the red, scaly, weepy lesions described 6
• It is often difficult to separate atopic dermatitis from seborrheic dermatitis in infancy, and the two conditions may overlap in this age group 1

Diagnostic Criteria Met

This patient fulfills the Taiwan Academy of Pediatric Allergy, Asthma and Immunology (TAPAAI) diagnostic criteria for pediatric atopic dermatitis: 1

1. Pruritus with age-specific distribution pattern - Present (itchy rash on scalp, face, trunk, extensor surfaces)
2. Eczema in infancy pattern - Present (starts on face/scalp, extends to trunk and extensor surfaces)
3. Duration >2 months for infants - Would need to be confirmed in history

Clinical Pitfall to Avoid

The most common diagnostic error is confusing early atopic dermatitis with seborrheic dermatitis in infants under 1 year of age. 1 The key distinguishing features are:

• Presence of significant pruritus (favors atopic dermatitis) 1
• Sparing of groin/axillae (favors atopic dermatitis) 1
• Weepy, eczematous lesions rather than greasy scales (favors atopic dermatitis) 1, 6