**Core Concept**
Iodine deficiency disorders (IDD) are a significant public health concern worldwide, particularly in areas with inadequate iodine intake. The World Health Organization (WHO) has established guidelines to assess the prevalence of IDD in communities. Goitre, an enlargement of the thyroid gland, is a key indicator of iodine deficiency.

**Why the Correct Answer is Right**
According to WHO guidelines, the prevalence of goitre in school-age children is a critical indicator of iodine deficiency. A goitre prevalence of 5% or more in school-age children is considered the threshold for IDD to be endemic in a community. This is because goitre is a visible manifestation of iodine deficiency and can be used as an indicator of the population's iodine status. The thyroid gland is responsible for regulating iodine levels in the body, and a deficiency can lead to goitre, hypothyroidism, and other related disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** A goitre prevalence of 2% is considered low and does not indicate IDD.
**Option B:** A goitre prevalence of 10% is considered high, but it is not the threshold for IDD to be endemic.
**Option C:** A goitre prevalence of 5% is the correct answer, but the statement is incomplete and lacks context.

**Clinical Pearl / High-Yield Fact**
The WHO recommends a goitre prevalence of less than 5% in school-age children as an indicator of adequate iodine intake in the population. This can be achieved through the use of iodized salt, which is a simple and effective strategy to prevent IDD.

**Correct Answer:** C. 5%