Lupus vulgaris, the most common form of cutaneous tuberculosis in India, is characterised by which pathognomic clinical sign?

• A Apple-jelly nodules on diascopy ✓
• B Undermined edges with blue-red discolouration (pyoderma-like)
• C Painless clean-based ulcer with indurated edges
• D Follicular keratotic papules with scales

Explanation

Lupus vulgaris is characterised by 'apple-jelly' nodules (yellowish-brown, jelly-like translucency) seen on diascopy (pressing a glass slide over the lesion). This appearance represents granulomatous infiltrates in the dermis. Histology shows well-formed tuberculous granulomas (epithelioid cells, Langhans giant cells) with or without caseation. Complications include destructive scarring, mutilation (rhinophyma-like nasal destruction), and development of squamous cell carcinoma in chronic lesions. Undermined blue-red edges characterise scrofuloderma. Clean-based ulcer with indurated edges suggests primary inoculation tuberculosis.

Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.

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