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) rather than resolving within 24 hours as expected with urticaria 1.
Lack of response to standard urticaria therapy (hydrocortisone and antihistamines) strongly argues against urticaria as the primary diagnosis 1, 2. True urticaria typically responds to H1-antihistamines and corticosteroids 2.
Fixed, non-migratory nature of individual lesions once established, though new lesions appeared in different locations. Urticarial wheals characteristically migrate and individual lesions resolve within 24 hours 1.
Likely trigger identification: Recent food exposure (string beans and egg) can trigger 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] |
• Initial presentation with pruritic wheals [2] • Erythematous, blanchable lesions • Pruritus present • Possible food trigger |
• Target-like lesions present [1] • Lesions persisted >24 hours [1] • No response to hydrocortisone or cetirizine [1,2] • Fixed rather than evanescent wheals [2] |
Initial clinical presentation mimicked urticaria, and urticaria multiforme (a variant) can present with polycyclic annular wheals that may be confused with target lesions [3,4]. The distinction requires observation of lesion evolution over time. |
| Erythema Multiforme Minor | • Target-like lesions (pathognomonic) [1] • Lesions persist and evolve over days [1] • Fixed lesions (non-migratory) [1] • Self-limited course [1] • Often triggered by infections or medications [1] |
• Target-like lesions documented [1] • Lesions persisted over days [1] • Spontaneous resolution after 2 days [1] • No response to urticaria therapy [1] • Recent food exposure as potential trigger [1] |
• No documented HSV or mycoplasma infection (common triggers) • Relatively rapid resolution (typically 1-2 weeks) |
This is the most likely primary diagnosis based on pathognomonic target lesions and clinical course [1]. However, skin biopsy showing interface dermatitis with keratinocyte necrosis would provide definitive confirmation [1]. |
| Viral Exanthem | • Associated with viral prodrome or systemic symptoms [4] • Maculopapular or morbilliform rash pattern • Fever often present • Self-limited course • Non-specific histology [1] |
• Undocumented fever present • Self-limited course • Mild leukocytosis (14.39 × 10⁹/L) suggesting reactive process • Age-appropriate for viral infections |
• Target-like lesions not typical of viral exanthem [1] • No viral prodrome (cough, colds, URI symptoms) documented • No exposure to persons with similar symptoms • Rapid onset without typical viral progression |
Cannot be completely ruled out because viral exanthems can have variable presentations [4], and the mild leukocytosis with neutrophil predominance could represent viral or post-viral response. Skin biopsy would show non-specific perivascular lymphocytic infiltrate rather than interface dermatitis [1]. |
| Serum Sickness-Like Reaction (SSLR) | • Urticarial rash with fever [4] • Arthralgia/arthritis • Lymphadenopathy • Occurs 7-21 days after medication/antigen exposure [4] • Elevated ESR/CRP • Purpuric component may be present [4] |
• Urticarial-appearing rash • Fever present • Possible food antigen exposure |
• No arthralgia or joint swelling documented • No lymphadenopathy on examination • No medication exposure documented • No purpuric component • Timing inconsistent (2 hours after meal, not 7-21 days) [4] • Target lesions not typical of SSLR [4] |
Cannot be entirely excluded without complete medication history and inflammatory markers (ESR, CRP) [4]. SSLR is a mimicker of urticaria in children and can present with fever and extracutaneous manifestations [4]. However, the presence of target lesions and absence of arthralgia/lymphadenopathy make this less likely. |
Additional Distinguishing Features and Diagnostic Approach
Clinical Pitfalls to Avoid:
Urticaria multiforme is a benign variant that can present with polycyclic annular wheals mimicking target lesions but lacks true target morphology with three distinct zones 3, 4. This entity is commonly misdiagnosed as EM or SSLR 3.
Individual urticarial lesions resolve within 24 hours, whereas EM lesions are fixed and evolve over days 1. Marking individual lesions can help distinguish between these conditions.
Lack of response to antihistamines and corticosteroids is a red flag against urticaria as the primary diagnosis 1, 2. Standard-dose second-generation H1-antihistamines lead to symptom resolution in fewer than 50% of true urticaria cases, but complete non-response suggests an alternative diagnosis 5.
Histopathologic Differentiation:
If diagnostic uncertainty persists, skin biopsy can definitively distinguish between these entities 1:
- EM: Interface dermatitis with keratinocyte necrosis and lymphocytic infiltrate 1
- Urticaria: Dermal edema and perivascular infiltrate without keratinocyte damage 1
- Viral exanthem: Variable non-specific perivascular lymphocytic infiltrate 1
Laboratory Findings Interpretation:
The mild leukocytosis (14.39 × 10⁹/L) with neutrophil predominance (61%) is consistent with a reactive process but is non-specific 1. This could support viral exanthem, post-infectious EM, or inflammatory response to allergen.
Mild microcytosis (MCV 73.8 fL) and hypochromia (MCH 23.9 pg) suggest possible iron deficiency or thalassemia trait but are unrelated to the acute presentation.
Normal eosinophil count (0%) argues somewhat against allergic urticaria, though eosinophilia is not always present in acute urticaria 2.
Management Implications:
EM Minor requires supportive care only with no specific treatment 1. The spontaneous resolution in this case supports this diagnosis.
Identification and avoidance of triggers is recommended 1. The temporal relationship with string beans and egg consumption suggests possible food trigger, though EM is more commonly associated with HSV and mycoplasma infections.
No need for systemic immunosuppression or escalation to omalizumab/cyclosporine as would be considered in refractory chronic urticaria 5.