## **Core Concept**
Tuberculids are skin eruptions that occur in patients with tuberculosis (TB), believed to result from an immune response to tuberculin or other products of *Mycobacterium tuberculosis*. They are not directly caused by the bacteria but are associated with the systemic effects of TB.

## **Why the Correct Answer is Right**
Erythema nodosum is indeed considered a type of tuberculid. It presents as an acute, inflammatory condition characterized by the sudden onset of tender, erythematous nodules, typically on the lower legs. It can be associated with TB among other causes like infections, medications, and sarcoidosis. On the other hand, **lupus vulgaris** is actually a form of cutaneous tuberculosis, not a tuberculid. It results from the direct inoculation of *M. tuberculosis* into the skin or from hematogenous or lymphatic spread from an internal focus of TB.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Erythema induratum (also known as Bazin disease) is considered a tuberculid. It presents as recurring painful, tender, indurated plaques and nodules on the calves of young women, often associated with TB.
- **Option B:** Papulonecrotic tuberculid typically presents with symmetrically distributed, necrotizing skin lesions that heal with scarring. It is associated with TB.
- **Option D:** Lichen scrofulosorum is a rare tuberculid that presents with lichenoid eruptions, often on the abdomen and thighs, associated with TB.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while tuberculids are associated with TB, they do not contain viable *Mycobacterium tuberculosis* bacteria. This differentiates them from true cutaneous TB, which does contain the bacteria.

## **Correct Answer:** B. lupus vulgaris