In a population of 5000, number of new cases of TB is 500; old cases in the same population are 150. What is the prevalence of TB:
**Core Concept**
Prevalence is a measure of the total number of cases of a disease in a population at a specific point in time, including both new and existing cases. It is an essential epidemiological parameter used to assess the burden of a disease.
**Why the Correct Answer is Right**
To calculate the prevalence of TB, we need to add the number of new cases (500) to the number of existing cases (150) and divide the total by the population size (5000). This gives us the ratio of people with TB to the total population, which is a measure of prevalence. The formula for prevalence is: Prevalence = (Number of new cases + Number of existing cases) / Total population.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it only considers the number of new cases and ignores the existing cases, which are also part of the prevalence.
**Option B:** This option is incorrect because it only considers the number of existing cases and ignores the new cases, which are also part of the prevalence.
**Option C:** This option is incorrect because it subtracts the number of existing cases from the number of new cases, which would give a negative value and does not make sense in the context of prevalence.
**Clinical Pearl / High-Yield Fact**
When calculating prevalence, it's essential to include both new and existing cases, as it provides a comprehensive picture of the disease burden in a population.
**Correct Answer: C. (500 + 150) / 5000 = 650 / 5000 = 0.13 or 13%**