What is the primary diagnosis and differential diagnoses for a patient with sudden onset of pruritic, erythematous, migratory rashes, and erythematous lesions on the scalp, face, back, abdomen, groin, arms, and legs, who was initially diagnosed with acute urticaria but did not respond to hydrocortisone and had resolution of symptoms after 2 days?

Primary Diagnosis: Erythema Multiforme Minor

The primary diagnosis is erythema multiforme (EM) minor, not acute urticaria, based on the fixed nature of the lesions lasting 2 days, lack of response to hydrocortisone, and the clinical evolution of the rash. 1

Key Distinguishing Features Supporting EM Minor

Lesion Characteristics That Favor EM Over Urticaria

• Fixed lesions: The rashes persisted for 2 days before resolution, whereas urticaria lesions typically resolve within 24 hours 1
• Non-migratory nature: Despite initial description as "migratory," the lesions remained fixed in location for days, which is characteristic of EM (lesions remain fixed for minimum 7 days) rather than true urticarial migration 1
• Distribution pattern: Widespread involvement of scalp, face, back, abdomen, groin, arms and legs is consistent with EM minor's typical distribution on trunk and extremities 2
• Lack of wheals on second presentation: The evolution from wheals (first visit) to erythematous, non-raised, blanchable macular rash (second visit) suggests progression to EM rather than persistent urticaria 1

Why Hydrocortisone Failed

• EM minor does not respond to corticosteroids: Studies demonstrate that steroid-treated EM patients show no better response than non-steroid treated groups, except for shorter fever duration 3
• Paradoxical reaction possible: Rare cases of corticosteroid-induced urticaria have been reported, though this is uncommon 4

Critical Differential Diagnoses

1. Viral Exanthem (Most Important to Rule Out)

Why it remains on the differential:

• Undocumented fever with rash onset 1
• Age-appropriate for common viral infections (roseola, enterovirus, parvovirus B19) 1
• Maculopapular distribution on trunk and extremities 1
• Mild leukocytosis (14.39 × 10⁹/L) consistent with viral infection

Distinguishing features needed:

• Viral exanthems typically have shorter duration (3-5 days) and different morphology
• Lack of target or iris lesions argues against EM but their absence doesn't exclude it 2
• Missing critical information: No documentation of target lesions, which are pathognomonic for EM 1, 2

2. Urticaria (Initial Working Diagnosis)

Why it was initially considered:

• First presentation showed wheals on face 1
• Acute onset after food exposure (string beans and egg)
• Pruritic nature

Why it should be ruled out:

• Lesions persisted beyond 24 hours 1
• Evolution to fixed, non-raised macular rash inconsistent with urticaria 1
• No response to antihistamines (cetirizine) over multiple days
• Second presentation lacked typical wheal formation

3. Drug Reaction (Less Likely But Consider)

Why to consider:

• Patient received paracetamol and cetirizine between visits
• Hydrocortisone exposure during hospitalization
• However, timing doesn't fit typical drug reaction pattern

4. Mycoplasma-Associated Rash (Important in This Age Group)

Why it's relevant:

• Mycoplasma pneumoniae is a significant trigger for EM, particularly common in children 1
• May present with predominantly mucous membrane involvement or cutaneous lesions 1
• Better prognosis compared to other EM causes 1

Missing information:

• No respiratory symptoms documented
• No mycoplasma testing performed

Critical Diagnostic Gaps in This Case

What Was NOT Documented (But Essential):

1. Lesion morphology details: No mention of target or iris lesions (concentric rings with dusky red and white centers) 1, 2
2. Lesion evolution tracking: Borders should have been marked with ink and photographed to document fixed vs. migratory nature 1
3. Mucous membrane examination: No detailed oral, conjunctival, or genital examination documented 2
4. Herpes simplex history: HSV is the most common trigger for EM, but no history obtained 2, 5
5. Recent infections: No testing for mycoplasma, HSV, or other viral triggers 1, 5

Why EM Minor is the Final Diagnosis

The discharge diagnosis of EM minor is correct based on:

• Clinical course: Self-limited resolution in 2 days is consistent with EM minor 2
• Lack of steroid response: Characteristic of EM, not urticaria 3
• Fixed lesion duration: Lesions persisted beyond the 24-hour window that defines urticaria 1
• Systemic symptoms: Fever with rash onset is common in EM 2
• Age and presentation: Consistent with pediatric EM minor 1

Recommended Diagnostic Approach for Future Similar Cases

Immediate Assessment:

• Mark lesion borders with ink and photograph to document progression over 24-48 hours 1
• Examine for target lesions: Look for concentric rings with central dusky red area surrounded by pale ring and outer erythematous ring 1, 2
• Complete mucous membrane examination: Oral cavity, conjunctiva, genitals 2
• Document lesion characteristics: Raised vs. flat, blanching, size, distribution 1

Laboratory Workup:

• HSV serology or PCR if recurrent episodes 5
• Mycoplasma pneumoniae testing (IgM, PCR) if respiratory symptoms or in endemic periods 1
• Skin biopsy if diagnosis uncertain: EM shows variable epidermal damage from individual cell apoptosis to confluent necrosis 1

Key Clinical Pearls:

• EM lesions remain fixed for ≥7 days; urticaria resolves within 24 hours 1
• Corticosteroids are not effective for EM minor and should not be used as diagnostic test 3
• Most EM is HSV-triggered; consider suppressive antiviral therapy if recurrent 5
• Mycoplasma-associated EM in children may have minimal cutaneous findings 1