A 52-year-old woman with acute myeloid leukemia presents with high fever (39.5°C), neutrophilic leukocytosis, and multiple painful lesions on her face, neck, and upper extremities. On examination, the structure marked **A** in the diagram shows tender violaceous edematous plaques with a pseudo-vesicular surface. A skin biopsy reveals dense dermal neutrophilic infiltrate without leukocytoclastic vasculitis. Which of the following best describes the pathogenic mechanism underlying the lesion marked **A**?

Explanation

## Why "Reactive cytokine-mediated dermatosis..." is right Sweet syndrome (acute febrile neutrophilic dermatosis) is fundamentally a reactive cytokine-mediated condition, not an infectious or vasculitic process. The tender edematous plaques marked **A** result from intense neutrophilic infiltration of the dermis driven by abnormal neutrophil chemotaxis and elevated G-CSF and IL-6 levels. The pseudo-vesicular appearance arises from papillary edema mimicking true vesicles, not from actual fluid-filled blisters. The absence of vasculitis on histology is a KEY diagnostic feature that distinguishes Sweet syndrome from other neutrophilic dermatoses. This pathogenic mechanism is the cornerstone of diagnosis and explains the dramatic response to systemic corticosteroids (Cohen Sweet Syndrome Review 2007; Bolognia Dermatology 4th ed). ## Why each distractor is wrong - **Primary leukocytoclastic vasculitis...**: Leukocytoclastic vasculitis is explicitly ABSENT in Sweet syndrome. The diagnostic hallmark is dense dermal neutrophilic infiltrate WITHOUT vasculitis. This is a critical distinguishing feature that rules out conditions like acute leukocytoclastic vasculitis. - **Direct infection of dermal tissue...**: Sweet syndrome is a sterile, non-infectious dermatosis. Although it may follow infection (classic subtype) or be drug-induced, the lesions themselves contain no organisms. The pathology is reactive, not infectious. - **Autoimmune destruction of dermal collagen...**: While Sweet syndrome can be associated with underlying malignancy or drugs, the primary mechanism is not autoimmune collagen destruction. The pathology is neutrophilic infiltration with edema, not collagen-targeted autoimmunity. **High-Yield:** Sweet syndrome = fever + tender edematous plaques + dermal neutrophils WITHOUT vasculitis + steroid response = reactive cytokine-mediated, NOT infectious or vasculitic. [cite: Cohen Sweet Syndrome Review 2007; Bolognia Dermatology 4th ed]