**Core Concept**
Prevalence is a measure used in epidemiology to describe the total number of cases of a disease or condition present in a population at a specific point in time. It is an indicator of the burden of disease within a population and is often used to track the impact of public health interventions.

**Why the Correct Answer is Right**
Prevalence is calculated as the total number of existing cases of a disease or condition divided by the total population at risk. This measure does not take into account the duration of the disease or the timing of new cases, making it a useful indicator for conditions with varying durations or for diseases with a high incidence rate. Prevalence is often used in conjunction with incidence to understand the dynamics of a disease within a population.

**Why Each Wrong Option is Incorrect**
**Option A:** Incidence is the rate of new cases of a disease or condition within a population over a specific period of time, not a measure of existing cases.
**Option B:** Morbidity refers to the state of being diseased or unhealthy, but it is not a specific measure of disease prevalence.
**Option C:** Mortality refers to the rate of deaths due to a disease or condition within a population, which is distinct from the measure of existing cases.

**Clinical Pearl / High-Yield Fact**
When interpreting prevalence rates, it's essential to consider the population being studied, as prevalence can vary significantly depending on the demographic characteristics of the population.

**Correct Answer: B. Morbidity.