## **Core Concept**
The annual parasite incidence (API) is a measure used to express the number of confirmed malaria cases per 1000 population per year. It is an important indicator for assessing the malaria situation in a given area. The API is calculated using the number of malaria-positive cases and the population at risk.

## **Why the Correct Answer is Right**
To calculate the API, we use the formula: API = (Number of confirmed malaria cases / Population at risk) * 1000. Given that there are 500 malaria-positive slides out of 20,000 examined, and assuming the examined population is representative of the total population of 100,000 (1 lakh), we can directly use the 500 positive cases as the numerator. The denominator (population at risk) is 100,000. Therefore, API = (500 / 100,000) * 1000 = 5.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If we assume this option provides a calculation not based on the correct formula or incorrect numbers, it would be incorrect without the actual value provided for comparison.
- **Option B:** Similarly, without the specific value, if it does not match the calculation of (500 / 100,000) * 1000, it is incorrect.
- **Option D:** This option would also be incorrect if its value does not align with the correct API calculation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the API is a crucial epidemiological tool for malaria control programs. It helps in identifying high-risk areas and in evaluating the effectiveness of interventions over time. An API of 5 means that there are 5 confirmed malaria cases per 1000 population per year.

## **Correct Answer:** . 5 per 1000 population per year