A 35-year-old man from rural Maharashtra presents with a single hypopigmented patch on his left forearm with diminished sensation. Slit-skin smear examination shows no acid-fast bacilli. Based on clinical and bacteriological findings, what is the most common classification of leprosy in this patient?

Explanation

## Classification of Leprosy: Ridley-Jopling System **Key Point:** Tuberculoid leprosy (TT) is the most common form of leprosy encountered in clinical practice, accounting for approximately 50–60% of all leprosy cases in endemic regions. ### Clinical Features of Tuberculoid Leprosy | Feature | Tuberculoid Leprosy | |---------|--------------------| | **Number of lesions** | 1–5 (typically solitary or few) | | **Sensory loss** | Early and marked | | **Acid-fast bacilli (AFB)** | Absent or scanty (0–1+ on slit-skin smear) | | **Nerve involvement** | Asymmetric; early and prominent | | **Immune response** | Strong cell-mediated immunity (CMI) | | **Skin lesion borders** | Well-defined, sharply demarcated | | **Hypopigmentation** | Marked | ### Why This Patient Has Tuberculoid Leprosy 1. **Single lesion** — characteristic of TT 2. **Negative slit-skin smear** — indicates paucibacillary disease (TT has <5 lesions and negative smear) 3. **Early sensory loss** — hallmark of strong CMI response 4. **Well-demarcated hypopigmented patch** — typical TT presentation **High-Yield:** Tuberculoid leprosy represents the **immunologically competent end** of the Ridley-Jopling spectrum. Patients mount a strong cell-mediated immune response, limiting bacterial multiplication and resulting in few lesions with high sensory loss. **Clinical Pearl:** The presence of a single lesion with sensory loss and negative bacteriology is pathognomonic for tuberculoid leprosy in an endemic area. ### Ridley-Jopling Classification Overview ```mermaid graph TD A[Leprosy Classification]:::outcome --> B[Tuberculoid TT]:::action A --> C[Borderline Tuberculoid BT]:::action A --> D[Mid-Borderline BB]:::action A --> E[Borderline Lepromatous BL]:::action A --> F[Lepromatous LL]:::outcome B --> G[1-5 lesions, AFB negative]:::outcome C --> H[Few lesions, AFB 1-2+]:::outcome F --> I[Innumerable lesions, AFB 4-6+]:::outcome ``` **Mnemonic:** **TT BB LL** — Remember the spectrum from **strong immunity** (TT: few lesions, no AFB) to **weak immunity** (LL: many lesions, heavy AFB load).