A young boy presented with lesion over his right buttock which had peripheral scaling and central scarring. The investigation of choice would be
This is a case of Lupus vulgaris. Lupus vulgaris is a rare, chronic, progressive form of cutaneous tuberculosis that develops as a result of direct extension from underlying joints or lymph nodes; through lymphatic or hematogenous spread; or, rarely, by cutaneous inoculation with BCG vaccine. It most commonly follows cervical adenitis or pulmonary tuberculosis. Approximately 33% of cases are preceded by scrofuloderma, and 90% of cases present on the head and neck, most commonly on the nose or cheek. Involvement of the trunk is uncommon. A typical solitary lesion consists of a soft, brownish-red papule that has an apple-jelly color when examined by diascopy. One or several lesions may develop, including nodules or plaques that are flat and serpiginous, hyperophic and verrucous, or edematous in appearance. Spontaneous healing occurs centrally, and lesions characteristically reappear within the area of atrophy. Lymphadenitis is present in 40% of those with lupus vulgaris. Squamous cell carcinoma, with a relatively high metastatic potential, may develop, usually after several years of the disease. After a temporary impairment in immunity, paicularly after measles infection (lupus exanthematicus), multiple lesions may form at distant sites as a result of hematogenous spread from a latent focus of infection. Histologic examination and a positive culture for M. tuberculosis/bovis confirm the diagnosis. The tuberculin test is strongly positive except for the early phases of post-exanthematic lupus. Acid-fast bacilli are usually not found on histopathological examination. Small lesions can be excised. Antituberculous drug therapy usually halts fuher spread and induces involution. Ref: 1) Kliegman: Nelson Textbook of Pediatrics, 18th ed; Chapter 664 - Cutaneous Bacterial Infections. 2) Fitzpatrick's Dermatology in General Medicine (6th edition)
Tzanck Smear
KOH preparation
Biopsy
Sabouraud's agar