A 12-year-old boy had a gradually progressive plaque on a buttock for the last 3 years. The plaque was 15 cm in diameter, annular in shape, with crusting and induration at the periphery and scarring at the centre. The most likely diagnosis –
**Core Concept**
Lupus vulgaris is a form of cutaneous tuberculosis, characterized by a chronic, progressive skin lesion with crusting, induration, and scarring. It is the most common form of cutaneous tuberculosis and typically presents as an annular plaque with central scarring.
**Why the Correct Answer is Right**
Lupus vulgaris results from the direct extension of tuberculosis from an underlying focus, such as a lymph node or bone, to the skin. The characteristic annular plaque with central scarring is due to the granulomatous inflammation and subsequent fibrosis. The presence of crusting and induration at the periphery is due to the active tuberculous infection. The lesion's gradual progression over 3 years is consistent with the chronic nature of lupus vulgaris.
**Option A:**
* **Option A:** Tinea corporis is a fungal infection, typically caused by Trichophyton or Microsporum species. It presents as a circular, annular plaque with central clearing, not crusting and induration. Tinea corporis does not cause scarring and is usually treated with antifungal agents.
**Option B:**
* **Option B:** Granuloma annulare is a benign skin condition characterized by multiple, small, annular lesions with a normal epidermis and a granulomatous dermal infiltrate. It does not cause crusting, induration, or scarring.
**Option D:**
* **Option D:** Borderline leprosy is a form of leprosy that presents with a mixture of tuberculoid and lepromatous features. It is characterized by multiple, small, skin lesions with nerve involvement, not a single, large, annular plaque with crusting and scarring.
**Clinical Pearl / High-Yield Fact**
Lupus vulgaris is often associated with underlying tuberculosis infection, particularly in regions with high TB prevalence. A high index of suspicion is essential for diagnosing cutaneous tuberculosis, as it may mimic other skin conditions.
**✓ Correct Answer: C. Lupus vulgaris**