**Question:** WHO recommended modern single drug therapy in paucibacillary leprosy is -

A. Rifampicin
B. Dapsone
C. Clofazimine
D. Methylene Blue

**Core Concept:**
Leprosy is a chronic infectious disease caused by Mycobacterium leprae or Mycobacterium lepromatosis. It primarily affects the skin, peripheral nerves, and mucosal tissues. The disease is classified into two forms: paucibacillary (PB) and multibacillary (MB). PB cases have fewer than 10 bacilli, while MB cases have more than 10 bacilli. PB cases are easier to treat due to fewer bacilli and better drug penetration.

**Why the Correct Answer is Right:**
The World Health Organization (WHO) recommends a modern single drug therapy for PB leprosy. This approach aims to reduce drug toxicity, cost, and the risk of drug resistance. Dapsone (option B) is the first-line drug for PB leprosy. It is a bacteriostatic antibiotic that inhibits DNA synthesis, preventing the growth of M. leprae. Dapsone is effective against both M. leprae and M. lepromatosis, which are the causative organisms for leprosy.

**Why Each Wrong Option is Incorrect:**

A. Rifampicin (option A) is a bacteriocidal antibiotic, meaning it kills the bacteria directly. However, it is not the first-line drug for PB leprosy.

B. Clofazimine (option C) is a bacteriostatic antibiotic like dapsone but is not the first-line drug in WHO guidelines. It is recommended for use in combination therapy with other drugs.

D. Methylene Blue (option D) is not a drug used in leprosy treatment. It is a dye used for the staining of cells and tissues, primarily used in pathology.

**Clinical Pearl:**
Dapsone is the cornerstone of modern leprosy therapy due to its bacteriostatic effect, effectiveness against both M. leprae and M. lepromatosis, and its low toxicity profile. The use of a single drug reduces the risk of drug resistance and simplifies treatment. The WHO recommends a standardized treatment protocol for PB leprosy using a combination of dapsone and either clofazimine or rifampicin.