**Core Concept**
Prevalence is a measure of the total number of cases of a disease or condition present in a population at a specific point in time or over a specified period. It is an essential epidemiological parameter used to assess the burden of disease in a population and to determine the health needs of a community.

**Why the Correct Answer is Right**
Prevalence measures all cases of a disease or condition, including both new and existing cases. This is because it takes into account the total number of individuals with the disease or condition at a given time, regardless of when they first developed it. Prevalence is not limited to new cases, unlike incidence, which measures the number of new cases occurring within a specified period. Prevalence provides a comprehensive understanding of the disease burden in a population, making it a crucial tool for healthcare planning and resource allocation.

**Why Each Wrong Option is Incorrect**
**Option A:** Prevalence can be used to determine the health needs of a community by providing information on the total number of cases of a disease or condition, which can inform healthcare resource allocation and planning.

**Option B:** Prevalence is not independent of incidence because it takes into account the total number of cases, including both new and existing cases, which are influenced by the incidence rate.

**Option C:** Prevalence is not independent of duration because the longer the duration of a disease or condition, the higher the prevalence, as more individuals will be affected.

**Clinical Pearl / High-Yield Fact**
A key difference between prevalence and incidence is that prevalence measures the total number of cases, including both new and existing cases, whereas incidence measures only new cases. Understanding this distinction is crucial for accurate epidemiological analysis and healthcare planning.

**Correct Answer:**
✓ Correct Answer: D. Measures all cases