A 22-year-old woman presents with painful red nodules on both anterior shins for the past 2 weeks. She reports a sore throat 3 weeks ago that resolved spontaneously. On examination, the lesions marked **A** in the diagram are tender, warm, and erythematous, with color changes from deep red to purple-brown. There is no ulceration or scarring. She is afebrile with normal vitals. Which of the following best describes the pathological process underlying these lesions marked **A**?

Explanation

## Why "Septal panniculitis without vasculitis, representing a hypersensitivity reaction to streptococcal antigen" is right The tender erythematous nodules marked **A** in erythema nodosum (EN) are histologically characterized by **septal panniculitis without vasculitis** — inflammation of the fibrous septa between fat lobules, not the blood vessels themselves. This is a hallmark feature of EN as described in Robbins 10e. The clinical presentation (post-streptococcal, bilateral pretibial nodules, color changes without ulceration, self-limited course) is classic for EN as a **hypersensitivity reaction pattern** to underlying triggers, most commonly group A streptococcal infection. The absence of vasculitis distinguishes EN from other panniculitides and vasculitic conditions. ## Why each distractor is wrong - **Acute suppurative inflammation with neutrophilic infiltration and bacterial vasculitis**: This describes acute bacterial infection or abscess formation, not the sterile hypersensitivity-driven septal panniculitis of EN. EN does not feature vasculitis or suppuration. - **Granulomatous inflammation with caseating necrosis and acid-fast bacilli**: This describes tuberculosis or other mycobacterial panniculitis. While TB is a known cause of EN, the histology would show granulomas and acid-fast bacilli, not the septal panniculitis pattern. The clinical context (post-strep, no respiratory symptoms) points to streptococcal EN, not TB. - **Lymphocytic vasculitis of medium-sized vessels with fibrinoid necrosis and immune complex deposition**: This describes polyarteritis nodosa or other systemic vasculitides. EN is explicitly **not** a vasculitis; the defining feature is septal inflammation without vessel involvement. **High-Yield:** Erythema nodosum = **septal panniculitis WITHOUT vasculitis** = hypersensitivity reaction (not a disease itself); most common identifiable cause is post-streptococcal infection; self-limited, resolves in 3–6 weeks without scarring. [cite: Robbins 10e Ch 25]