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
• 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:

1. Continue symptomatic management with antihistamines (cetirizine already started) 2, 3
2. Add topical corticosteroids to affected areas for symptomatic relief 2, 3
3. Start empiric macrolide antibiotic (azithromycin 10 mg/kg/day for 5 days) while awaiting Mycoplasma testing, given high suspicion in this age group 3

Monitor for Complications:

• Assess oral mucosa carefully - Mycoplasma-associated EM can present with predominantly mucous membrane involvement 1
• Ensure adequate hydration and nutrition - severe mucosal involvement may require hospitalization 2, 3
• Watch for progression - document lesion evolution with photographs 1

Common Pitfalls to Avoid

Do not dismiss this as simple urticaria or viral exanthem without considering EM - the target-like lesions are diagnostic 1, 2

Do not delay Mycoplasma testing - this is the one EM trigger that requires specific antibiotic treatment 1, 3

Do not confuse with SJS - EM has raised target lesions predominantly on extremities, while SJS has flat atypical targets with epidermal detachment 1