**Core Concept:** Prevalence is calculated as the number of cases already present in a given population at a specific point in time, considering both incidence (new cases) and duration of illness.

**Why the Correct Answer is Right:** Prevalence (P) can be calculated using the formula: P = (Incidence * Mean duration of disease) / 1000

In this question, the incidence is stated as 50 cases per 1000 population per year, and the mean duration of the disease is 5 years. When plugging these values into the formula, we get:

P = (50 * 5) / 1000 = 250 / 1000 = 0.25 cases/1000 population

**Why Each Wrong Option is Incorrect:**

A. **Incorrect calculation**: This option provides a wrong formula or incorrect values, leading to an incorrect prevalence calculation.
B. **Incorrect calculation**: Similar to option A, this choice contains a wrong formula or incorrect values, leading to an incorrect prevalence calculation.
C. **Incorrect calculation**: This option also has a wrong formula or incorrect values, resulting in an inaccurate prevalence estimation.
D. **Same calculation**: This option correctly applies the formula but uses the same calculated prevalence value as the correct answer, which is incorrect since the question asks for the prevalence rate (number of cases per 1000 population).

**Clinical Pearl:** To calculate prevalence, ensure you use the correct formula and provide the prevalence rate (number of cases per 1000 population). In clinical practice, prevalence is crucial for understanding the burden of disease, resource allocation, and planning healthcare interventions.

**Correct Answer:** 0.25 cases/1000 population (0.25 per 1000)