Lupus vulgaris (the most common form of cutaneous tuberculosis in India) shows which pathognomonic clinical sign on diascopy?

• A Scarlet-red blanchable erythema
• B Golden-yellow follicular plugs
• C Brown-red non-blanchable macules
• D Salmon-colored papules ('apple jelly' nodules) ✓

Explanation

Diascopy (pressing a glass slide on the skin) of lupus vulgaris reveals 'apple jelly' nodules — translucent yellow-brown/salmon-pink color corresponding to the granulomatous infiltrate in the dermis. This is pathognomonic of lupus vulgaris (reinfection/high immunity cutaneous TB). The lesions are soft, slowly progressive, preferentially affect the face (nose, ear), and can cause scarring and squamous cell carcinoma in longstanding disease.

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

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