**Core Concept:** Prevalence of a disease refers to the number of people affected by the disease at a specific point in time, given the incidence and duration of the disease. Prevalence can be calculated using the formula: Prevalence = (Incidence x Duration) / 1000.

**Why the Correct Answer is Right:** To calculate the prevalence, we use the formula: Prevalence = (Incidence x Duration) / 1000. In this case, the incidence is 4 cases per 1000 of population per year and the duration is 2 years. Plugging these values into the formula, we get: Prevalence = (4 x 2) / 1000 = 8 / 1000.

**Why Each Wrong Option is Incorrect:**

A. Prevalence = (6 x 3) / 1000 (Incorrect because the duration is incorrectly chosen as 3 years)
B. Prevalence = (2 x 4) / 1000 (Incorrect because the duration is incorrectly chosen as 4 years)
C. Prevalence = (5 x 2) / 1000 (Incorrect because the duration is incorrectly chosen as 5 years)
D. Prevalence = (3 x 2) / 1000 (Incorrect because the duration is incorrectly chosen as 3 years)

**Clinical Pearl:** Understanding the difference between incidence and prevalence is crucial in epidemiology. Incidence refers to the number of new cases occurring in a defined population during a specific period, while prevalence represents the total number of cases present in a population at a given moment, considering both new cases and existing cases.

**Correct Answer:** Prevalence = (4 x 2) / 1000 = 8 / 1000. The correct answer is D, as the duration is correctly chosen as 2 years, which is given in the question.