Lupus vulgaris (LV) is a form of post-primary, chronic, and paucibacillary cutaneous tuberculosis in individuals with a high tuberculin sensitivity and moderate immunity. It is the most common type of cutaneous tuberculosis.
Cutaneous tuberculosis (TB) can be caused by M. tuberculosis, M. bovis, and the Bacillus Calmette–Guérin (BCG) vaccine. The disease has diverse clinical features that can follow immune-mediated processes or endogenous and exogenous spread of the microorganism. The exogenous spread is less common. Even though cutaneous TB comprises less than 1% of all TB cases, it is important to consider it when a clinical picture suggestive of it is found . Exogenous inoculation develops after M. tuberculosis is directly inoculated into the skin of an individual susceptible to the infection. This results in tuberculosis verrucosa cutis (TVC), TB chancre, and in some cases lupus vulgaris (LV). Endogenous spread occurs in previously infected cases by means of contiguous extension, hematogenous dissemination or through lymphatic circulation or in old scars of scrofuloderma . Hematogenous spread is observed in LV, acute miliary TB, papulonecrotic tuberculoid (PNT), and metastatic TB abscess (gummatous TB). A contiguous extension can be seen in orificial TB and scrofuloderma.
LV is the most common type of cutaneous TB and affects females 2 to 3 times more often than males. It is chronic and progressive.The lesions of LV are commonly found in the region of the head and neck  and are generally solitary, small, sharply defined, nodular, reddish brown, with a gelatinous consistency (apple jelly nodules). In western countries, these lesions usually appear on the head and neck while in tropical or subtropical regions they develop on lower extremities and the buttocks.
The existing clinical variations are as follows:
Persistent lesions become ulcerated and damage the underlying structures leading to disfigurement and have an increased risk of malignant transformation.
The diagnosis of LV can be a challenging task as the skin changes are atypical and there is a paucity of TB bacilli in the lesions.
Skin biopsy and histology may show tubercules or tuberculoid granulomas without caseation in the papillary dermis with few or no bacilli, and variable epidermal hyperplasia  . If a superficial biopsy is obtained then pseudo-epithelial hyperplasia with a non-specific inflammatory cell infiltrate may be observed, missing the features of LV .
Identification of Mycobacteria is essential for the definitive diagnosis of cutaneous TB but this is usually impossible since there are few bacilli. M. tuberculosis rarely grows on culture when the sample is taken from a patient with high levels of immunity or from chronic lesions.
Special tests like the polymerase chain reaction (PCR) can be performed to confirm the diagnosis or when the histopathological findings are not characteristic of the condition .
Ziehl-Neelsen staining and PCR may also give negative results in LV due to the paucity of TB bacilli  .
Diagnosis relies heavily on clinical features, histopathological findings, a positive purified protein derivative test (PPD) and response to anti-TB medication  .