Influenza-Associated Urticaria Multiforme Mimicking Serum Sickness in a Toddler: A Diagnostic Challenge.

Urticaria multiforme (UM) is a benign hypersensitivity reaction of early childhood characterized by transient annular urticarial plaques with dusky centers and acral edema. Although self-limited, UM is frequently misdiagnosed as serum sickness-like reaction (SSLR), erythema multiforme (EM), or urticarial vasculitis due to overlapping features such as fever, edema, and inflammatory laboratory abnormalities. Diagnostic uncertainty may therefore lead to extensive investigations, hospitalization, and unnecessary systemic corticosteroid therapy. A three-year-old male with a history of atopic dermatitis presented with decreased oral intake, lethargy, fever, and intermittent erythematous wheals associated with swelling of the feet and extremities. Initial evaluation revealed influenza A (H3) infection, thrombocytosis, and mildly elevated C-reactive protein. Extensive diagnostic evaluation, including multiple imaging studies and laboratory investigations, was largely unremarkable except for preserved complement levels. Due to concern for a serum sickness-like reaction, systemic corticosteroids were initiated. The dermatology consultation subsequently favored urticaria multiforme based on the transient nature of lesions lasting less than 24 hours, absence of mucosal involvement or palpable purpura, preserved complement levels, and progressive clinical improvement. The patient was managed with antihistamines and supportive care, with complete resolution of fever, edema, and urticarial lesions prior to discharge after a hospital course of six days. This case highlights influenza-associated urticaria multiforme presenting with fever and mild inflammatory marker elevation, closely mimicking immune complex-mediated disease. Recognition of lesion duration, morphology, and complement preservation is essential for accurate diagnosis and to prevent unnecessary immunosuppressive therapy.