A 28-year-old woman presents with a 2-week history of painful oral ulcers and a characteristic rash on her palms and extensor surfaces of the forearms. Examination reveals multiple lesions with three distinct concentric zones: a central dusky vesicular area (marked **A**), surrounded by a pale edematous ring, and an outer erythematous border. She recalls a cold sore 3 weeks prior. Which of the following best explains the pathophysiology of the central dusky/necrotic zone marked **A**?

Explanation

## Why Type III hypersensitivity reaction with immune complex deposition is right Erythema multiforme is a classic Type III hypersensitivity reaction (immune complex-mediated) triggered most commonly by HSV infection (70-90% of recurrent EM minor cases). The central dusky/necrotic zone (**A**) represents epidermal damage resulting from immune complex deposition in dermal vessels and the dermal-epidermal junction, leading to complement activation, inflammation, and keratinocyte necrosis. This is NOT direct viral invasion but rather a host immune response to viral or other antigens. The three-zone target lesion architecture (central necrosis, pale edema, outer erythema) is pathognomonic for EM and reflects this hypersensitivity mechanism. Robbins 10e Ch 25 emphasizes EM as a hypersensitivity reaction with immune complex involvement in the pathogenesis. ## Why each distractor is wrong - **Direct viral invasion of keratinocytes by HSV with lytic cell death**: While HSV is the most common trigger of EM, the lesions of EM do NOT result from direct viral lysis. HSV acts as an antigen triggering immune complexes, not as a direct cytopathic agent in the target lesion itself. This would be the mechanism of primary herpetic infection, not EM. - **Type IV delayed hypersensitivity with CD8+ T-cell mediated epidermal cytotoxicity**: Although T-cell involvement occurs in EM, the primary pathophysiology is Type III (immune complex), not Type IV. Type IV hypersensitivity predominates in drug-induced reactions like SJS/TEN, which are distinct from EM and show full-thickness epidermal necrosis rather than the limited central zone necrosis of EM. - **Bacterial superinfection with exotoxin-mediated tissue destruction**: EM is not caused by bacterial exotoxins. While secondary bacterial infection can complicate EM lesions, it is not the primary mechanism of the central necrotic zone. The lesions are sterile immune-mediated phenomena. **High-Yield:** EM is a **Type III hypersensitivity** (immune complex) reaction; the three-zone target lesion reflects dermal inflammation and epidermal damage from immune complexes—NOT direct pathogen invasion. HSV is the trigger, not the direct cause of tissue damage. [cite: Robbins 10e Ch 25; Nelson 21e Ch 671]