Primary Diagnosis: Erythema Multiforme Minor
The primary diagnosis is Erythema Multiforme (EM) Minor, not acute urticaria, based on the presence of target-like lesions, persistence of rashes beyond 24 hours despite antihistamine therapy, and spontaneous resolution over 2 days without response to hydrocortisone. 1
Rationale for EM Minor as Primary Diagnosis
The key diagnostic features that distinguish this case as EM Minor include:
Target-like lesions are pathognomonic for EM and are not characteristic of urticaria 1. The patient explicitly had "(+) SOME TARGET LIKE LESIONS" documented on examination.
Temporal evolution of lesions that persisted and evolved over days (initial presentation on 10/18, recurrence 1 day prior to 11/03 consult, then resolution after 2 hospital days) is consistent with EM Minor rather than urticaria, where individual lesions resolve within 24 hours 1.
Lack of response to hydrocortisone suggests a non-histamine-mediated process, which is more consistent with EM than urticaria 1.
Fixed, non-migratory quality of the rashes (described as "erythematous, non-raised, blanchable macular rash" with some target-like lesions) differs from the transient, migratory wheals of urticaria 2.
Recent food exposure (string beans and egg) can serve as a potential trigger for EM Minor, and the American Academy of Dermatology recommends identification and avoidance of such triggers 1.
Differential Diagnoses: Comparative Analysis
| Diagnosis | Key Diagnostic Criteria | Features Present in This Patient | Features Absent/Against Diagnosis | Why It Cannot Be Ruled Out Yet |
|---|---|---|---|---|
| Acute Urticaria | • Pruritic wheals that resolve within 24 hours [1,2] • Individual lesions blanch completely [1] • No target morphology [1] • Responds to antihistamines [2,3] |
• Initial presentation with "erythematous wheals on face" [2] • Pruritic nature of rash [2] • Blanchable lesions [2] |
• Presence of target-like lesions [1] • Persistence beyond 24 hours [1] • No response to hydrocortisone or cetirizine [1] • Fixed rather than evanescent lesions [2] |
Initial presentation mimicked urticaria with wheals and pruritus [4]. Urticaria multiforme, a benign variant, can present with polycyclic annular wheals that mimic EM [4,5]. The lack of complete resolution with antihistamines within 24 hours argues against classic urticaria [1]. |
| Erythema Multiforme Minor | • Target-like lesions (pathognomonic) [1] • Lesions persist and evolve over days [1] • Fixed distribution [1] • Supportive care only, no specific treatment [1] • Skin biopsy shows interface dermatitis with keratinocyte necrosis [1] |
• Target-like lesions documented [1] • Persistence over multiple days [1] • Spontaneous resolution after 2 days [1] • No response to hydrocortisone [1] • Recent food exposure as potential trigger [1] |
• No skin biopsy performed to confirm histopathology [1] • Mild fever present (typically EM Minor has minimal systemic symptoms) |
This is the most likely diagnosis [1]. However, without skin biopsy confirmation showing interface dermatitis with keratinocyte necrosis and lymphocytic infiltrate, definitive diagnosis remains clinical [1]. The presence of target lesions is highly specific [1]. |
| Viral Exanthem | • Associated with viral prodrome or concurrent viral illness [5] • Non-specific maculopapular rash [5] • Systemic symptoms (fever, malaise) [5] • Skin biopsy shows variable non-specific perivascular lymphocytic infiltrate [1] |
• Undocumented fever present [5] • Mild leukocytosis (14.39 × 10⁹/L) suggesting reactive process • Age-appropriate for viral infections [5] • Self-limited course [5] |
• No clear viral prodrome (no cough, colds, or URI symptoms) • Presence of target-like lesions not typical for viral exanthem [1] • No exposure to persons with similar symptoms |
Viral infections are common triggers for both urticaria and EM in children [5]. The mild leukocytosis and fever could represent a viral process. Without specific viral testing or clear viral syndrome, this cannot be definitively excluded [5]. The target-like lesions make pure viral exanthem less likely [1]. |
| Serum Sickness-Like Reaction (SSLR) | • Urticarial rash with fever [5] • Arthralgias/arthritis [5] • Occurs 7-21 days after medication or antigen exposure [5] • Polycyclic or annular urticarial plaques [5] • Extracutaneous manifestations common [5] |
• Urticarial-appearing rash initially [5] • Fever present [5] • Recent food exposure (potential antigen) [5] |
• No documented medication exposure • No arthralgias or joint symptoms [5] • No lymphadenopathy [5] • Timing (2 hours after meal) too rapid for SSLR [5] • No extracutaneous manifestations [5] |
SSLR is commonly misdiagnosed when urticarial rashes present with fever in children [4,5]. The absence of arthralgias, lymphadenopathy, and appropriate temporal relationship to antigen exposure makes this less likely [5]. However, atypical presentations can occur, and without complete exclusion of recent medication exposure, it remains a differential [5]. |
Critical Distinguishing Features by Diagnosis
Acute Urticaria
- Hallmark: Individual wheals that are transient (resolve within 24 hours) and intensely pruritic 2
- Response to treatment: First-line therapy with second-generation H1 antihistamines (cetirizine) should show improvement 2, 3
- Morphology: Wheals blanch completely, no target morphology 1
- This patient: Had initial wheals but developed target-like lesions and failed antihistamine therapy 1
Erythema Multiforme Minor
- Hallmark: Target-like lesions (pathognomonic) with central dusky area, intermediate pale zone, and peripheral erythema 1
- Duration: Lesions persist and evolve over days, not hours 1
- Treatment response: Does not respond to antihistamines or corticosteroids; requires supportive care only 1
- This patient: Matches all criteria with documented target lesions, persistence over days, and lack of response to hydrocortisone 1
Viral Exanthem
- Hallmark: Non-specific maculopapular rash associated with viral prodrome or concurrent viral illness 5
- Histopathology: Variable non-specific perivascular lymphocytic infiltrate without keratinocyte damage 1
- This patient: Had fever and leukocytosis but lacked viral prodrome and had atypical target-like lesions 1, 5
Serum Sickness-Like Reaction
- Hallmark: Urticarial rash with fever PLUS extracutaneous manifestations (arthralgias, lymphadenopathy) occurring 7-21 days post-exposure 5
- Morphology: Polycyclic or annular urticarial plaques, often misdiagnosed as urticaria multiforme 4, 5
- This patient: Lacked arthralgias, lymphadenopathy, and appropriate temporal relationship to antigen exposure 5
Clinical Pitfalls and Caveats
Urticaria multiforme is commonly misdiagnosed as EM or SSLR 4, 5. However, urticaria multiforme should respond to antihistamines within 24 hours, which did not occur in this case 4.
The presence of target-like lesions is the most critical distinguishing feature 1. Even one true target lesion shifts the diagnosis toward EM rather than urticaria 1.
Skin biopsy is the gold standard for definitive differentiation: interface dermatitis with keratinocyte necrosis confirms EM, while dermal edema and perivascular infiltrate without keratinocyte damage confirms urticaria 1.
Corticosteroids are not indicated for EM Minor and lack of response to hydrocortisone supports this diagnosis 1. In contrast, urticaria may show some response to corticosteroids as adjunctive therapy 2.
The mild microcytosis (MCV 73.8 fL) and hypochromia (MCH 23.9 pg) suggest possible iron deficiency anemia, which is unrelated to the acute presentation but warrants follow-up evaluation.