A 28-year-old woman presents with tender, warm, erythematous nodules on the anterior shins bilaterally that appeared over the past 2 weeks. She reports fever, arthralgias, and recent sore throat treated with antibiotics. On examination, the lesions are 2–3 cm, non-ulcerating, and non-scarring. Skin biopsy confirms the diagnosis. The underlying pathology marked **D** in the diagram—reactive septal panniculitis—indicates that erythema nodosum is fundamentally a hypersensitivity reaction. Which of the following is the MOST important next step in management of this patient?

Explanation

## Why option 1 is correct Erythema nodosum (EN) is a **reactive septal panniculitis**—a hypersensitivity reaction to an underlying trigger, not a primary skin disease. The clinical anchor is that EN **requires workup to identify and treat the underlying cause**. In this patient, the history of recent sore throat is a red flag for **streptococcal infection**, the most common identifiable cause of EN globally and in India. Obtaining an ASO titer and throat culture directly identifies the trigger, allowing targeted antibiotic therapy, which addresses the root cause and promotes resolution. Per Harrison 21e Ch 70, streptococcal pharyngitis is the KEY association requiring serology and culture confirmation. Management of EN hinges on finding and treating the underlying disease—not just symptom control. ## Why each distractor is wrong - **Option 2 (Start prednisolone immediately)**: While corticosteroids are used for severe or refractory EN, they must be used cautiously and ONLY after infectious causes—especially tuberculosis—are ruled out. Starting steroids empirically without workup risks masking or worsening TB, a major cause of EN in India. Steroids are not first-line management; identifying the cause is. - **Option 3 (Potassium iodide)**: SSKI is effective for refractory or idiopathic EN (when no cause is found after thorough workup), not for initial management when a clear trigger (streptococcal infection) is evident. It is reserved for cases unresponsive to cause-directed therapy. - **Option 4 (Compression and NSAIDs alone)**: Supportive measures are adjunctive but insufficient. Without identifying and treating the streptococcal trigger, the nodules will persist or recur. Symptomatic relief alone ignores the fundamental principle that EN is a marker of underlying disease requiring investigation. **High-Yield:** Erythema nodosum is **not a diagnosis—it is a sign of systemic disease**. Always workup for the cause (strep, TB, IBD, sarcoidosis, malignancy) before escalating to steroids or immunosuppression. [cite: Harrison 21e Ch 70]