What are the differential diagnoses for a patient with pruritic, erythematous, migratory rashes and a primary diagnosis of erythema multiforme minor, and what are their distinguishing features?

Differential Diagnoses for Erythema Multiforme Minor

Primary Working Diagnosis Assessment

The primary diagnosis of erythema multiforme minor is questionable in this case because the patient lacks the pathognomonic target or "iris" lesions with well-defined concentric rings, and the rash is described as non-raised, blanchable, and migratory—features inconsistent with classic EM. 1, 2

Key Differential Diagnoses

1. Acute Urticaria (Most Likely)

This remains the most probable diagnosis given the clinical presentation:

• Distinguishing features present:

  • Pruritic, erythematous, migratory rashes 2
  • Non-raised, blanchable lesions
  • Rapid onset and spread pattern
  • Previous episode documented 2 weeks prior
  • Individual lesions in urticaria typically resolve within 24 hours, whereas EM lesions remain fixed for minimum 7 days 2

• Why it cannot be ruled out:

  • The migratory nature strongly suggests urticaria rather than EM 2
  • Absence of fixed target lesions makes EM less likely 1, 3
  • Associated fever could represent concurrent viral infection triggering urticaria
  • Mild leukocytosis (14.39 × 10⁹/L) supports reactive process

2. Viral Exanthem

Cannot be excluded based on current presentation:

• Distinguishing features present:

  • Undocumented fever with rash onset 4
  • Maculopapular distribution on trunk and extremities
  • Age-appropriate for common viral infections (roseola, enterovirus, parvovirus B19) 4
  • Mild leukocytosis with normal differential

• Why it cannot be ruled out:

  • No specific viral testing performed
  • Rash pattern could fit multiple viral etiologies 4
  • Temporal association with fever suggests infectious trigger
  • Common viral exanthems can present with pruritus

3. Drug Hypersensitivity Reaction

Must be considered given medication exposure:

• Distinguishing features present:

  • Recent administration of cetirizine and paracetamol
  • Timing of rash persistence after medication initiation
  • Drugs can precipitate EM-like reactions 3, 5

• Why it cannot be ruled out:

  • Although cetirizine is an antihistamine, paradoxical reactions can occur
  • Paracetamol rarely causes hypersensitivity but cannot be completely excluded
  • No rechallenge performed to confirm or exclude drug causation

4. Erythema Multiforme Minor

Less likely but cannot be completely excluded:

• Features that would support EM:

  • Target lesions with central clearing and concentric rings (NOT present) 1, 3
  • Fixed lesions for minimum 7 days (NOT documented) 2
  • Predominant distribution on extremities, especially extensor surfaces (partially present) 2
  • Preceding HSV or Mycoplasma infection (not documented) 1, 6

• Why it cannot be definitively ruled out:

  • Early EM may not show classic target morphology initially 3
  • Atypical presentations can occur, particularly in young children 6
  • However, the non-raised, blanchable, migratory nature strongly argues against EM 1, 2

5. Mycoplasma-Associated Rash

Should be considered given age and presentation:

• Distinguishing features:

  • Mycoplasma pneumoniae is common trigger in children 1, 6
  • Can present with variable rash patterns 4
  • May have predominantly mucous membrane involvement 1

• Why it cannot be ruled out:

  • No respiratory symptoms documented, but subclinical infection possible
  • No Mycoplasma serology or PCR performed
  • Rash pattern not specific enough to exclude

6. Tinea Corporis (Less Likely)

Should be considered but less probable:

• Features that would support tinea:

  • Well-demarcated erythematous plaques with scaling borders (NOT clearly described) 7
  • Prominent advancing border with central clearing (NOT present) 7

• Why it cannot be ruled out:

  • No KOH preparation or fungal culture performed 7
  • Widespread distribution makes fungal infection less likely
  • Acute onset argues against dermatophyte infection

Critical Missing Diagnostic Information

To definitively differentiate these conditions, the following are needed:

• Detailed lesion morphology: Are true target lesions with three distinct zones present? 1, 3
• Lesion duration: Do individual lesions remain fixed >7 days or resolve within 24 hours? 2
• Mucosal involvement: Any oral, conjunctival, or genital lesions? 6, 8
• Infectious workup: Mycoplasma serology/PCR, HSV PCR if mucosal lesions present 1, 6
• Medication timeline: Exact timing of drug administration relative to rash onset
• Lesion evolution: Photograph and mark borders to document progression or migration 4

Common Pitfalls to Avoid

• Do not diagnose EM without true target lesions with well-defined concentric rings 1, 3
• Do not confuse migratory urticarial lesions with fixed EM lesions 2
• Do not overlook drug causation—review ALL medications including over-the-counter agents 3, 5
• Do not miss Mycoplasma infection—consider testing even without respiratory symptoms in children 1
• Do not assume previous "acute urticaria" diagnosis was correct—recurrent episodes warrant broader differential 2