A 7-year-old child with untreated primary pulmonary tuberculosis develops asymptomatic, tiny, skin-coloured, grouped lichenoid papules in a cluster over the trunk. The Mantoux test is strongly positive. Biopsy shows superficial granulomas in the dermal papillae. Acid-fast staining of the lesions is expected to be:
- A Negative or showing very few bacilli, because it is a hypersensitivity reaction to tuberculous antigen ✓
- B Positive, because lichen scrofulosorum contains abundant bacilli
- C Positive only in immunocompromised children
- D Negative, because the eruption is unrelated to tuberculosis
Correct answer: A. Negative or showing very few bacilli, because it is a hypersensitivity reaction to tuberculous antigen
Explanation
Lichen scrofulosorum is a tuberculid: a hypersensitivity response to mycobacterial antigen in a child with active tuberculosis elsewhere, usually the lungs or lymph nodes. It appears as grouped minute lichenoid papules, often on the trunk, with granulomas confined to the dermal papillae. Like all true tuberculids, AFB are absent or extremely sparse, and the lesions respond to antitubercular therapy directed at the primary focus. The strong Mantoux supports the diagnosis.
Reference: Rook's Textbook of Dermatology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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