A 24-year-old woman presents with the clinical finding shown in the diagram. The structure marked **A** (tender erythematous nodules) on the anterior shins is noted to be painful, initially deep red in color, and shows progressive color changes to yellow and green over the following weeks without ulceration. She reports a sore throat 3 weeks ago. Which of the following best describes the pathological nature of the lesion marked **A**?

Explanation

## Why "Septal panniculitis without vasculitis representing a hypersensitivity reaction pattern" is right Erythema nodosum (EN) is the most common form of panniculitis, characterized histologically by SEPTAL PANNICULITIS (inflammation in the connective tissue septae of subcutaneous fat) WITHOUT vasculitis. Critically, EN is NOT a disease entity itself but rather a hypersensitivity reaction pattern triggered by underlying causes — in this case, likely post-streptococcal (sore throat 3 weeks prior). The tender erythematous nodules marked **A** represent this pattern, which is self-limited and resolves over 3–6 weeks with characteristic bruise-like color changes (red → yellow → green → brown) without ulceration or scarring. (Robbins 10e Ch 25; Harrison 21e Ch 354) ## Why each distractor is wrong - **Lobular panniculitis with vasculitis**: Lobular panniculitis occurs in the fat lobules themselves (not septae) and is seen in conditions like erythema induratum (tuberculosis-associated) or nodular vasculitis. EN is specifically SEPTAL panniculitis without vasculitis. - **Nodular vasculitis with fat necrosis requiring urgent systemic corticosteroids**: Nodular vasculitis (erythema induratum) is a different entity with vasculitis and fat necrosis, typically associated with TB. EN does not have vasculitis. Systemic steroids are reserved only for severe/refractory cases after infectious causes are ruled out. - **Acute suppurative infection of subcutaneous tissue requiring antibiotic therapy**: EN is a sterile hypersensitivity reaction, not an infection. While the trigger may be streptococcal pharyngitis, the nodules themselves are not suppurative and do not require antibiotics directed at the nodules (though the underlying strep infection should be treated). **High-Yield:** Erythema nodosum = septal panniculitis (no vasculitis) = hypersensitivity reaction pattern (not a disease) triggered by identifiable or idiopathic causes; workup focuses on finding the trigger, not treating the nodules as primary pathology. [cite:Robbins 10e Ch 25; Harrison 21e Ch 354]