## **Core Concept**
The question tests understanding of the levels of prevention in public health, specifically where screening of diseases falls within these levels. The levels of prevention include primary, secondary, tertiary, and sometimes quaternary prevention. Screening for diseases is a crucial strategy that aims to detect diseases early.

## **Why the Correct Answer is Right**
Screening of diseases is considered a part of **secondary prevention**. Secondary prevention aims to detect and treat problems early before the disease manifests or becomes severe. It involves screening tests that can identify diseases in their earliest stages, often before symptoms appear, thereby enabling early intervention. This can include activities like mammography for breast cancer, blood pressure checks for hypertension, and blood glucose tests for diabetes.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Primary prevention involves measures to prevent the occurrence of disease. It includes interventions aimed at avoiding the development of risk factors or preventing the onset of disease. Screening for diseases does not fit here because it does not prevent the disease but detects it early.
- **Option C:** Tertiary prevention aims to reduce the impact of an ongoing illness or injury that has lasting effects. It focuses on helping manage and rehabilitate patients with established disease. While important, this does not involve screening for diseases but rather managing them after diagnosis.
- **Option D:** Quaternary prevention refers to actions taken to mitigate or avoid the consequences of unnecessary or excessive interventions within the health system. Screening for diseases does not fall under this category as it is not about avoiding over-intervention but about early detection.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that screening tests are crucial for **secondary prevention**. They should be applied to populations at risk for a disease, aiming to detect the disease early, and thereby improve outcomes. Examples include the Pap smear for cervical cancer and newborn screening for phenylketonuria (PKU).

## **Correct Answer:** . Secondary.