Lichen scrofulosorum: a rare clue to Mycobacterium tuberculosis infection

Cutaneous tuberculosis is a rare manifestation of extra-pulmonary tuberculosis and is subclassified into two categories: tuberculids and true cutaneous forms of tuberculosis. Tuberculids develop due to a hypersensitivity reaction after exposure to Mycobacterium tuberculosis or its fragments. Lichen scrofulosorum, one type of tuberculid, is characterized by dispersed asymptomatic skin-coloured to reddish-brown papules. When present, true mycobacterial infection is frequently associated. A strongly positive tuberculin skin test and the absence of acid-fast bacilli in skin smears are common tuberculid diagnostic features. The current antituberculous regimens, used in other manifestations of tuberculosis, appear to be equally effective. The authors report the rare case of a young patient presenting with lichen scrofulosorum, which proved to be essential for the definitive diagnosis of tuberculosis lymphadenitis.