A 35-year-old man from rural Maharashtra presents with a single hypopigmented macule on his left forearm with loss of sensation. On examination, the lesion is well-demarcated and anesthetic. Slit-skin smear examination shows no acid-fast bacilli (AFB). Which is the most appropriate next investigation to confirm the diagnosis and classify the type of leprosy?

Explanation

## Investigation of Choice to Confirm and Classify Leprosy **Key Point:** In a patient with a single hypopigmented anesthetic macule and negative slit-skin smear (paucibacillary leprosy), **nerve biopsy with histopathology** (or skin biopsy histopathology) is the most appropriate investigation to **confirm the diagnosis and classify** the type of leprosy — not the lepromin test. ### Why Nerve Biopsy / Histopathology? The question specifically asks for an investigation to **confirm the diagnosis AND classify** the type of leprosy. Histopathology of a skin or nerve biopsy achieves both goals simultaneously: | Feature | Tuberculoid (TT) | Lepromatous (LL) | |---------|-----------------|-----------------| | **Histopathology** | Well-formed epithelioid granulomas with Langhans giant cells; nerve destruction; few/no bacilli | Foamy (Virchow) macrophages; poorly formed granulomas; numerous bacilli (globi) | | **Nerve involvement** | Perineural granulomas; nerve destruction | Nerve infiltration by foamy macrophages | | **AFB on SSS** | Negative | Numerous (BI 4–6+) | | **Lepromin test** | Positive | Negative | **High-Yield:** The Ridley-Jopling classification of leprosy (TT, BT, BB, BL, LL) is based on **histopathological criteria** — the granuloma pattern, degree of nerve destruction, and bacillary index on biopsy. This is the gold standard for classification per Rook's Dermatology and IAL Textbook of Leprosy. ### Why NOT Lepromin Test? The lepromin (Mitsuda) test is **not a diagnostic test** — it measures the host's cell-mediated immune response to killed M. leprae antigen. It: - Cannot confirm the diagnosis of leprosy (it can be positive in healthy individuals who have been exposed) - Is used as an **immunological marker** to assess prognosis and immune status - Is **not recommended** as a primary investigation for classification in standard clinical practice (IAL guidelines) ### Why NOT PCR or EMG? - **PCR** is useful for detecting M. leprae DNA but does not classify the type histologically - **EMG** assesses nerve conduction but does not confirm or classify leprosy **Clinical Pearl:** Per IAL Textbook of Leprosy (3rd ed.) and Rook's Dermatology, skin biopsy (or nerve biopsy when skin biopsy is non-contributory) with histopathology is the definitive investigation for both confirming leprosy and classifying it along the Ridley-Jopling spectrum. **Mnemonic:** **Biopsy = Both confirm + classify**; Lepromin = Immunity marker only. [cite: IAL Textbook of Leprosy 3rd ed.; Rook's Dermatology 9th ed.; Ridley DS & Jopling WH, Int J Lepr 1966]