Scrofuloderma is a form of cutaneous tuberculosis caused by which mechanism, and which test most accurately distinguishes it from other cutaneous TB forms?

• A True skin infection via inoculation (prosector's wart); PCR for MTB from skin biopsy
• B Haematogenous dissemination (lupus vulgaris type); strongly positive Mantoux
• C Contiguous spread from underlying lymph node/bone TB; positive culture for MTB from discharge ✓
• D Hypersensitivity reaction to MTB antigens (lichen scrofulosorum); negative smear

Explanation

Scrofuloderma results from direct contiguous spread of TB from an underlying focus — most commonly cervical lymph nodes (scrofula), but also bone, joints, or epididymis — to the overlying skin, producing a discharging sinus/ulcer. Culture of MTB from the discharge remains definitive; PCR can support the diagnosis when culture is negative. Prosector's wart (tuberculosis verrucosa cutis) arises from exogenous inoculation. Lupus vulgaris is the most common form in Asia by haematogenous spread.

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

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