Erythema Multiforme — Target Lesions

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**?