A 42-year-old woman with a 3-year history of ulcerative colitis presents with a rapidly enlarging, exquisitely painful ulcer on her left shin. On examination, the lesion marked **A** in the diagram shows a distinctive violaceous, undermined, overhanging border with a purulent necrotic base. Tissue culture is negative for bacterial, fungal, and mycobacterial pathogens. Which of the following best describes the pathophysiological mechanism underlying this condition?

Explanation

## Why "Dysregulated neutrophil chemotaxis with abnormal T-cell function and overproduction of pro-inflammatory cytokines (IL-1, IL-8, IL-17, TNF-α)" is right Pyoderma gangrenosum (marked **A**) is a rare neutrophilic dermatosis, not an infectious condition despite its name. The pathogenesis involves dysregulated neutrophil chemotaxis, abnormal T-cell function, and overproduction of pro-inflammatory cytokines including IL-1, IL-8, IL-17, and TNF-α. The negative tissue culture confirms the sterile nature of the ulcer, ruling out infectious etiologies. This immune-mediated mechanism explains why the lesion is exquisitely painful and rapidly progressive, and why it responds to immunosuppressive therapy rather than antibiotics. (Bolognia Dermatology 5e Ch 26; STOP-GAP trial Lancet 2015) ## Why each distractor is wrong - **Direct bacterial invasion with production of exotoxins causing tissue necrosis and vasculitis**: The negative tissue culture explicitly rules out bacterial infection. PG is sterile by definition and is not caused by bacterial pathogens or their toxins, despite the misleading name "pyoderma." - **Thrombosis of dermal and subcutaneous blood vessels leading to ischemic necrosis**: While vascular involvement may occur secondarily, the primary pathophysiology of PG is neutrophilic inflammation and dysregulated immunity, not primary vasculitis or thrombosis. This mechanism better describes arterial or venous ulcers (marked **C** or **B**). - **Abnormal keratinocyte apoptosis triggered by autoimmune attack on basement membrane antigens**: This describes autoimmune blistering diseases such as pemphigus or bullous pemphigoid, not PG. PG is characterized by neutrophilic infiltration and leukocytoclasis, not primary keratinocyte or basement membrane pathology. **High-Yield:** PG = sterile, painful, violaceous undermined border + neutrophilic dermatosis (NOT infection) + 50–70% associated with IBD/RA — treat with immunosuppression, never debride. [cite: Bolognia Dermatology 5e Ch 26; STOP-GAP trial Lancet 2015]