## **Core Concept**
The World Health Organization (WHO) has specific guidelines for the control and treatment of trachoma, a leading cause of preventable blindness. Trachoma is caused by *Chlamydia trachomatis* and its prevalence is often assessed in children aged 1-9 years. The presence of trachomatous inflammation — follicular (TF) is a key indicator.

## **Why the Correct Answer is Right**
The correct answer, **10%**, is based on WHO guidelines for the control of trachoma. According to these guidelines, mass treatment with azithromycin is recommended in districts where the prevalence of trachomatous inflammation — follicular (TF) in children aged 1-9 years is 10% or greater. This approach aims to reduce the reservoir of infection in the community, thereby decreasing transmission and the risk of complications.

## **Why Each Wrong Option is Incorrect**
- **Option A: 5%**: This is below the threshold recommended by the WHO for mass treatment. While it's an important level of prevalence indicating the need for intervention, it does not meet the criteria for mass treatment with azithromycin.
- **Option B: 20%**: Although a high prevalence, the specific threshold for mass treatment is 10%, not 20%.
- **Option D: 30%**: Similar to option B, this is above the threshold but not the correct answer based on the specified criteria.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the decision for mass drug administration (MDA) with azithromycin for trachoma control is based on the prevalence of trachomatous inflammation — follicular (TF) in 1-9 year olds. The threshold of **10%** TF is critical for this decision, making it a high-yield fact for exams and clinical practice.

## **Correct Answer: B. 10%**