## Core Concept
The relationship between incidence (I), prevalence (P), and duration (D) of a disease is a fundamental concept in epidemiology. It helps in understanding the dynamics of disease spread and persistence in a population. The formula that connects these three parameters is crucial for calculating and predicting disease burden.

## Why the Correct Answer is Right
The correct formula that relates incidence (I), prevalence (P), and duration (D) is **P = I × D**. This formula implies that prevalence is equal to the product of the incidence rate and the average duration of the disease. Incidence (I) refers to the number of new cases that develop in a specified time period among a population at risk. Prevalence (P) is the total number of cases of the disease in the population at a given time. Duration (D) is the average length of time that an individual has the disease. This relationship holds under stable conditions, where the population is not changing, and the rates of disease onset and resolution are constant.

## Why Each Wrong Option is Incorrect
- **Option A:** This option does not accurately represent the relationship between incidence, prevalence, and duration. The correct relationship involves multiplication, not division or addition.
- **Option B:** This option incorrectly suggests that incidence is equal to the product of prevalence and duration, which reverses the actual relationship.
- **Option D:** This option implies a relationship where incidence is equal to prevalence divided by the square of duration, which is not accurate.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **P = I × D** can help in understanding how changes in disease duration (due to improved treatment, for example) can affect prevalence without a change in incidence. For instance, if the duration of a disease increases due to better treatment, the prevalence will increase even if the incidence remains constant.

## Correct Answer Line
**Correct Answer: C. P = I × D**