**Core Concept**
Trachoma is a chronic keratoconjunctivitis caused by the bacterium *Chlamydia trachomatis*, leading to blindness if left untreated. Early detection and treatment are crucial in preventing long-term complications. The World Health Organization (WHO) recommends a simplified grading system for trachoma screening.

**Why the Correct Answer is Right**
The correct answer is based on the WHO simplified grading system, which includes five signs: TF (trachomatous inflammation - follicular), TI (trachomatous inflammation - intense), TS (trachomatous scarring), CO (corneal opacity), and FO (follicular or intense trachomatous inflammation in at least 10% of the population aged 5-15 years). The correct answer is the key indicator for screening, which is the presence of **TF (Trachomatous Inflammation - Follicular)** in at least 10% of the population aged 5-15 years.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not a valid indicator for trachoma screening according to the WHO simplified grading system.
* **Option B:** Not a primary indicator for trachoma screening; TS (Trachomatous Scarring) is a sign of long-term damage.
* **Option D:** Not a relevant indicator for trachoma screening; other signs and symptoms may be present but are not the primary indicators.

**Clinical Pearl / High-Yield Fact**
The WHO simplified grading system is a crucial tool for trachoma control programs worldwide. Identifying **TF (Trachomatous Inflammation - Follicular)** in at least 10% of the population aged 5-15 years is a key indicator for trachoma prevalence and helps guide public health interventions.

**Correct Answer: C. TF (Trachomatous Inflammation - Follicular) in at least 10% of the population aged 5-15 years.**