## **Core Concept**
Prevalence of a disease is defined as the total number of cases of the disease in a population at a given time. It is a measure of the total burden of a disease in a population. Prevalence takes into account both new and existing cases.

## **Why the Correct Answer is Right**
To calculate prevalence, we need to know the total number of cases (new and existing) in the population at a specific point in time or over a specified period. The incidence rate given is 100 cases per year in a population of 5000. The duration of the disease studied is 2 years.

The **point prevalence** at the beginning or end of the study period isn't directly calculable with the given data, but we can estimate **period prevalence** which essentially reflects the proportion of the population that had the disease at any point during the 2-year period.

Given:
- Incidence rate = 100 cases/year
- Population = 5000
- Duration = 2 years

Assuming the incidence rate remains constant:
- Total new cases over 2 years = 100 cases/year * 2 years = 200 cases

However, to calculate prevalence directly, we typically need a measure of existing cases plus new cases over a period or at a point. Here, we might approximate but let's directly assess the provided choices.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without specific details on how it's calculated, we can't directly refute, but let's see other options.
- **Option B:** Similarly, lacks context for refutation.
- **Option D:** This seems to suggest a much higher prevalence than what might be expected with an incidence of 100 over 2 years in a population of 5000.

## **Clinical Pearl / High-Yield Fact**
A key formula in epidemiology is: **Prevalence = (Incidence x Duration) + Existing cases**. If we assume no one dies or recovers (which might not be realistic), and given incidence (100/year) and duration (2 years), without existing cases, prevalence would roughly equal 200 (new cases) / 5000 (population) * 100 for a simple proportion, but actual prevalence calculation needs more specific data on disease duration and recovery.

## **Correct Answer:** B. 120/5000.