A 35-year-old man from rural Maharashtra presents with a single hypopigmented macule on his forearm with diminished sensation. Slit-skin smear is negative for acid-fast bacilli. Which single clinical feature best distinguishes tuberculoid leprosy from borderline tuberculoid leprosy?

## Distinguishing Tuberculoid from Borderline Tuberculoid Leprosy **Key Point:** The cardinal discriminator between tuberculoid (TT) and borderline tuberculoid (BT) leprosy is the **number and distribution of skin lesions**. TT presents with 1–5 well-defined, asymmetric lesions; BT presents with 5–20 lesions that are more numerous and less sharply demarcated. ### Comparison Table | Feature | Tuberculoid (TT) | Borderline Tuberculoid (BT) | | --- | --- | --- | | **Number of lesions** | 1–5 | 5–20 | | **Lesion definition** | Sharp, well-demarcated | Less sharply demarcated | | **Symmetry** | Asymmetric | Asymmetric to mildly symmetric | | **Nerve involvement** | Single nerve, marked thickening | Multiple nerves, variable thickening | | **Bacillary load (BI)** | 0–1+ | 2–4+ | | **Cell-mediated immunity** | Strong (Mitsuda +ve) | Moderate (Mitsuda +ve) | | **Histology** | Epithelioid granulomas, few bacilli | Epithelioid granulomas, moderate bacilli | **High-Yield:** In **TT**, the immune response is so robust that lesions are few, well-demarcated, and bacilli are sparse (BI 0–1+). In **BT**, immunity is weaker, allowing more lesions and higher bacillary load (BI 2–4+). ### Why Lesion Count Matters The Ridley-Jopling classification (the gold standard) uses lesion morphology and number as a primary discriminator because: - **TT** = strong cell-mediated immunity → few lesions, low bacillary load - **BT** = intermediate immunity → more lesions, higher bacillary load - **Mid-borderline (BB)** = unstable immunity → many lesions, high bacillary load - **Borderline lepromatous (BL)** = weak immunity → numerous lesions, very high bacillary load - **LL** = absent immunity → innumerable lesions, massive bacillary load **Clinical Pearl:** A patient with 3 well-defined lesions and negative slit-skin smear is almost certainly TT. A patient with 12 lesions and positive smear is BT or beyond. **Mnemonic:** **TT = Tiny in number, Tremendous immunity** (1–5 lesions, BI 0–1+). **BT = Bigger in number, Blunted immunity** (5–20 lesions, BI 2–4+).