A 28-year-old woman from Tamil Nadu presents with a single well-demarcated hypopigmented patch with clear-cut borders and complete loss of sensation over the extensor surface of the left elbow. Slit-skin smear is negative for AFB. Which investigation is most appropriate to confirm the diagnosis and classify this lesion?

## Investigation of Choice for Tuberculoid Leprosy Diagnosis and Classification **Key Point:** Skin biopsy with histopathology is the gold standard for confirming tuberculoid leprosy (TT) and differentiating it from other hypopigmented conditions, especially when AFB is negative on slit-skin smear. ### Clinical Presentation Analysis The patient presents with: - Single well-demarcated lesion with clear-cut borders → **tuberculoid pattern** - Complete loss of sensation → **strong host immunity** - Negative AFB on slit-skin smear → **low bacillary load (paucibacillary)** - These features are classic for **Tuberculoid Leprosy (TT)** ### Why Histopathology is Essential Here **High-Yield:** In paucibacillary (TT/BT) disease, AFB may be negative on slit-skin smear, making histopathology the definitive diagnostic test. Histopathology in TT shows: 1. **Well-formed epithelioid granulomas** with Langhans giant cells 2. **Epidermal atrophy** over the granulomas 3. **Perineural inflammation** (pathognomonic for leprosy) 4. **Rare or absent AFB** (0–1 per field) 5. **Caseation necrosis** may be present ### Differential Diagnosis Ruled Out by Histology | Condition | Histology | AFB | |-----------|-----------|-----| | **Tuberculoid Leprosy** | Epithelioid granulomas, perineural inflammation | Rare/absent | | **Vitiligo** | Loss of melanocytes, no inflammation | Negative | | **Indeterminate Leprosy** | Non-specific inflammation, few granulomas | Negative | | **Morphea (localized scleroderma)** | Dermal fibrosis, no granulomas | Negative | **Clinical Pearl:** Perineural inflammation is the hallmark of leprosy histology and is absent in other hypopigmented conditions. This single finding can distinguish leprosy from vitiligo or other dermatoses. ### Why Other Tests Are Insufficient - **Lepromin test:** Positive in TT but does not confirm diagnosis; takes 3–4 weeks for result - **Nerve palpation:** Supports diagnosis but is subjective and non-specific; does not classify disease - **Serology (anti-PGL-I):** Negative in TT (paucibacillary); useful only in LL (lepromatous) [cite:Park 26e Ch Leprosy; Robbins 10e Ch Infectious Diseases]