Cutaneous infection due to exogenous exposure to TB bacilli in a person who is already sensitized by previous exposure to TB leads to:
**Core Concept**
Cutaneous tuberculosis (TB) refers to the infection of the skin caused by Mycobacterium tuberculosis. This condition can result from exogenous exposure to TB bacilli in individuals who are already sensitized by previous exposure. Cutaneous TB can manifest in various forms, depending on the route of infection and the immune status of the individual.
**Why the Correct Answer is Right**
TB verrucosa cutis is a form of cutaneous TB that occurs due to exogenous exposure to TB bacilli in a person who is already sensitized. This condition typically affects the skin overlying bony prominences, such as the elbows, knees, or ankles. The infection is characterized by the formation of verrucous lesions, which are warty, rough growths on the skin. The pathogenesis of TB verrucosa cutis involves the inoculation of TB bacilli into the skin, where they multiply and cause a localized inflammatory response.
**Why Each Wrong Option is Incorrect**
**Option A:** Lupus vulgaris is a form of cutaneous TB that occurs due to endogenous reactivation of latent TB infection, rather than exogenous exposure. It typically affects the face and is characterized by the formation of reddish-brown lesions.
**Option B:** TB chancre is a primary cutaneous lesion that occurs at the site of initial TB infection, typically on the skin overlying the site of inoculation. It is not associated with exogenous exposure to TB bacilli in a previously sensitized individual.
**Option C:** Erythema induratum is a form of cutaneous TB that occurs in the context of tuberculosis miliaris disseminata, a condition characterized by the development of multiple small, painful nodules on the lower legs. It is not specifically associated with exogenous exposure to TB bacilli.
**Clinical Pearl / High-Yield Fact**
TB verrucosa cutis is a form of cutaneous TB that can be confused with other skin conditions, such as psoriasis or eczema. A key differentiating feature is the presence of acid-fast bacilli in the skin lesions, which can be demonstrated by histopathological examination or culture.
**✓ Correct Answer: D. TB verrucosa cutis**