**Core Concept**
Iodine deficiency disorders (IDD) are a public health concern worldwide, particularly in areas with inadequate iodine intake. The World Health Organisation (WHO) has established criteria to define the prevalence of IDD in a community. The criteria are based on the prevalence of goiter in school-age children, which is a sensitive indicator of iodine deficiency.

**Why the Correct Answer is Right**
The WHO guidelines define iodine deficiency as a public health problem if the prevalence of goiter in school-age children is more than 10%. This is because goiter is a visible sign of iodine deficiency, and its presence in a community indicates a high risk of IDD. The prevalence of goiter is used as a proxy measure for iodine intake, as it reflects the long-term effect of iodine deficiency on thyroid function. School-age children are used as the reference population because they are most susceptible to the effects of iodine deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** 1% is too low to indicate a public health problem, as it may represent a normal variation in thyroid size.
**Option B:** 5% is still within the acceptable range, and goiter prevalence of 5% or less is considered to be a normal or low-risk indicator for IDD.

**Clinical Pearl / High-Yield Fact**
Mnemonically, the WHO criteria for defining iodine deficiency disorders can be remembered as: "10% or more goiter in school-age children indicates a public health problem".

**Correct Answer:**
✓ Correct Answer: C. 10%