## **Core Concept**
The question revolves around the risk of malignancy in a discrete thyroid swelling that is identified as a cystic lesion on neck ultrasonography (USG). Thyroid cysts are fluid-filled structures within the thyroid gland and can be either simple (benign) or complex (potentially malignant). The assessment of malignancy risk in such cases is crucial for guiding further management.

## **Why the Correct Answer is Right**
The risk of malignancy in a thyroid cyst is generally low but varies depending on the characteristics of the cyst. For isolated cystic thyroid lesions, especially those that are purely cystic with no solid components, the risk of malignancy is reported to be very low, typically less than 1%. However, the presence of solid components, septations, or suspicious features on USG increases the risk. Given that this lesion is described as an "isolated cystic swelling," the assumption here is that it is likely benign with a very low risk of malignancy, around 0-5%.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If this option suggests a higher risk (e.g., >20%), it would be incorrect for a purely cystic lesion without specifying other risk factors.
- **Option B:** A moderate risk (e.g., around 10-20%) might be considered for complex cysts or those with suspicious features, not purely cystic lesions.
- **Option D:** If this option suggests a very high risk (e.g., >50%), it would be overly concerning for a case described simply as an isolated cystic swelling without additional alarming features.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while the risk of malignancy in purely cystic thyroid lesions is low, the evaluation often involves fine-needle aspiration cytology (FNAC) for lesions that are solid, have suspicious features, or are large. For purely cystic lesions, especially if they are asymptomatic and do not have concerning features, active surveillance might be considered.

## **Correct Answer:** C. 5%