A patient on multidrug therapy for borderline lepromatous leprosy develops multiple tender erythematous subcutaneous nodules over the shins, fever, arthralgia and bilateral eye pain. Skin smear shows fragmented bacilli. Which immune mechanism best explains this episode?

• A Type IV hypersensitivity against Mycobacterium leprae antigens in existing lesions
• B Direct toxic effect of dapsone causing small vessel necrosis
• C Immune complex deposition with complement activation and neutrophilic vasculitis ✓
• D Cell-mediated downgrading shift towards the lepromatous pole

Explanation

Erythema nodosum leprosum (type 2 reaction) is a circulating immune complex mediated vasculitis occurring in lepromatous and borderline lepromatous disease, presenting with fever, crops of tender nodules, arthritis, iritis, neuritis and orchitis. Thalidomide is the classic drug for severe ENL. Option A describes a type 1 (reversal) reaction, which presents as swelling and ulceration of pre-existing plaques without systemic constitutional features, and that distinction kills the most tempting distractor.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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