A 32-year-old man from rural Maharashtra, diagnosed with lepromatous leprosy 6 months ago, presents with sudden onset of painful nodules on his extensor surfaces, fever (38.5°C), and tender nodular lesions on his legs. He has been compliant with multidrug therapy (MDT). Histopathology of the nodule shows neutrophilic infiltration with fibrinoid necrosis and immune complex deposition. What is the most likely diagnosis?

Explanation

## Diagnosis: Erythema Nodosum Leprosum (ENL) **Key Point:** ENL is a Type 2 lepra reaction (immune complex-mediated, Type III hypersensitivity) that occurs in lepromatous (LL) and borderline lepromatous (BL) leprosy patients, typically 6 months to 2 years after starting MDT or even during treatment. **Clinical Features of ENL:** - Painful, tender nodules on extensor surfaces (shins, forearms, elbows) - Systemic symptoms: fever, malaise - Acute onset during or shortly after MDT initiation - Can involve eyes (iritis), testes, nerves, and kidneys **Histopathology:** - Neutrophilic infiltration with fibrinoid necrosis - Immune complex (IgG, IgM, C3) deposition in dermal vessels - Distinguishes ENL from Type 1 reaction (which shows epithelioid granulomas) **High-Yield:** ENL occurs in **LL and BL** patients (high bacillary load), whereas Type 1 reactions occur in **BT, BL, and BB** (unstable forms). The presence of immune complex deposition on histology is pathognomonic for ENL. **Management:** - Continue MDT - NSAIDs for mild cases - Corticosteroids (prednisolone 0.5–1 mg/kg) for moderate-to-severe ENL - Thalidomide (50–100 mg daily) is highly effective for ENL (teratogenic; use with caution) **Clinical Pearl:** ENL can occur even years after completion of MDT and is a sign of immune reconstitution in lepromatous patients, not treatment failure.