A 32-year-old man from rural Maharashtra presents with a single hypopigmented macule on his left forearm with loss of sensation. Slit-skin smear is negative. Which feature best distinguishes tuberculoid leprosy from borderline tuberculoid leprosy?

## Distinguishing Tuberculoid from Borderline Tuberculoid Leprosy ### Clinical Context Both tuberculoid (TT) and borderline tuberculoid (BT) leprosy sit at the immunologically strong end of the Ridley-Jopling spectrum. The question asks for the **best distinguishing feature** between TT and BT — i.e., a feature that differs between the two types. ### Key Differentiating Feature **Key Point:** The **number of skin lesions** is the most reliable clinical discriminator between TT and BT leprosy. TT presents with a **single** (or at most 1–3) well-defined, sharply marginated hypopigmented macule/plaque, whereas BT presents with **a few to several** (typically 3–10) lesions that are less sharply defined. ### Comparison Table | Feature | Tuberculoid (TT) | Borderline Tuberculoid (BT) | |---------|-----------------|----------------------------| | **Skin lesions** | **1 (single)**, well-defined, sharp margins | **Few to several (3–10)**, less well-defined | | **Nerve involvement** | One or few nerves, asymmetric | Multiple nerves, asymmetric, more extensive | | **Slit-skin smear** | Negative (BI = 0) | Negative to scanty (BI = 0–1+) | | **Lepromin test** | Strongly positive | Weakly positive | | **Histology** | Epithelioid granuloma reaching epidermis, no bacilli | Epithelioid granuloma, scanty bacilli | ### Why Option A is Correct Per Ridley-Jopling classification (as cited in *Rook's Textbook of Dermatology* and *IAL Textbook of Leprosy*), the **number of skin lesions** is the primary clinical criterion used to separate TT (single lesion) from BT (few lesions). This is also the basis of the WHO operational classification: **paucibacillary single-lesion** (TT) vs. **paucibacillary 2–5 lesions** (BT). ### Why the Other Options Are Incorrect - **Option B (nerve thickening and asymmetry):** Both TT and BT show asymmetric nerve thickening; this feature does not reliably distinguish between them. - **Option C (AFB load):** Both TT and BT are typically smear-negative (BI = 0); scanty positivity in BT is inconsistent and not the primary distinguishing criterion. - **Option D (multiple peripheral nerves with symmetric thickening):** Symmetric nerve thickening is a feature of lepromatous (LL) leprosy, not BT. BT nerve involvement is asymmetric. This option does not correctly characterize either TT or BT. **Clinical Pearl:** The vignette itself illustrates TT — a **single** hypopigmented macule with sensory loss and negative smear. If there were 3–5 such lesions, the diagnosis would shift toward BT. **High-Yield (IAL / Ridley-Jopling):** TT = **1 lesion**; BT = **few lesions (2–5)**; BB = several; BL = many; LL = numerous/diffuse.