What specific viral exanthem is likely causing the patient's symptoms, given the presence of target-like lesions and erythema multiforme?

Erythema Multiforme Secondary to Mycoplasma pneumoniae or Herpes Simplex Virus

Based on the presence of target-like lesions, fever, and the patient's age (1 year 11 months), this presentation is most consistent with erythema multiforme (EM), with Mycoplasma pneumoniae and herpes simplex virus (HSV) being the two most likely viral/infectious triggers. 1, 2, 3

Most Likely Specific Viral Exanthem: Mycoplasma-Associated Erythema Multiforme

Given the patient's age, fever, and target lesions with mucosal involvement potential, Mycoplasma pneumoniae-associated erythema multiforme is the most likely diagnosis and must be ruled out immediately because it requires antibiotic treatment. 1, 3

Key Clinical Features Supporting This Diagnosis:

• Target-like lesions: The presence of "some target-like lesions" is pathognomonic for EM, distinguishing it from simple urticaria 1, 2
• Age group: Mycoplasma pneumoniae is particularly common in children and has a better prognosis compared to other EM causes 1
• Fever with rash: The undocumented fever occurring with rash onset is consistent with Mycoplasma-induced EM 1
• Fixed lesions: Unlike urticaria where individual lesions resolve within 24 hours, EM lesions remain fixed for a minimum of 7 days 1, 2

Critical Differential: HSV-Associated Erythema Multiforme

Herpes simplex virus is the most common infectious trigger for EM overall, but typically presents with a history of preceding HSV infection. 2, 3, 4

• HSV-associated EM usually follows a herpes outbreak by 3-14 days 3, 4
• No documented history of oral or cutaneous HSV lesions in this case makes this less likely as the primary trigger 3

Immediate Diagnostic and Management Steps

Rule Out Mycoplasma Pneumoniae First:

1. Obtain Mycoplasma pneumoniae serology (IgM and IgG) and PCR if available - this is critical because Mycoplasma requires antibiotic treatment 1, 3
2. Chest X-ray - to evaluate for pneumonia even if respiratory symptoms are minimal, as Mycoplasma can present with extrapulmonary manifestations 1
3. HSV serology and PCR - to evaluate for HSV as a trigger 3, 4

Why This Matters for Morbidity and Mortality:

Mycoplasma pneumoniae-associated EM requires antibiotic treatment (typically macrolides like azithromycin), whereas HSV-associated EM is managed symptomatically unless recurrent. 3 Missing Mycoplasma means missing the opportunity for etiologic treatment that can shorten disease duration and prevent complications.

Key Distinguishing Features from Other Conditions

Not Simple Urticaria:

• Urticarial lesions migrate and individual lesions resolve within 24 hours 1, 2
• This patient has fixed lesions present for >1 day with target morphology 1

Not Stevens-Johnson Syndrome (SJS):

• SJS presents with flat atypical targets or purpuric macules with epidermal detachment and positive Nikolsky sign 1, 2
• This patient has raised target lesions on extremities without described epidermal detachment 1
• EM has better prognosis and lower mortality than SJS 1

Not Other Viral Exanthems (Roseola, Enterovirus, Parvovirus):

• These typically present with maculopapular rashes without target lesions 1
• The presence of target-like lesions is specific for EM 1, 2

Management Approach

Immediate Treatment:

• Continue symptomatic management with antihistamines (cetirizine already started) 2, 3
• Consider topical corticosteroids for pruritus control 2
• Start empiric macrolide antibiotic (azithromycin) if Mycoplasma testing is delayed and clinical suspicion is high, particularly if any respiratory symptoms develop 3

Monitoring:

• Document lesion evolution by photographing and marking borders to confirm they remain fixed (not migratory like urticaria) 1
• Monitor for mucosal involvement - Mycoplasma-associated EM can present with predominantly mucous membrane involvement 1
• Ensure adequate hydration and monitor for severe mucosal involvement that might require hospitalization 2

Common Pitfalls to Avoid

1. Dismissing as simple urticaria without recognizing target lesions - this delays diagnosis of EM 1, 2
2. Failing to test for Mycoplasma pneumoniae - this is the only EM trigger requiring specific antibiotic treatment 1, 3
3. Confusing with SJS - EM has raised targets on extremities while SJS has flat atypical targets with epidermal detachment 1, 2
4. Not considering skin biopsy if diagnosis remains uncertain - histopathology shows variable epidermal damage and helps exclude other conditions 1