**Core Concept**
Incidence rate is a measure of the occurrence of new cases of a disease or event within a population over a specified period. Various special incidence rates are used to describe the risk of developing a disease in specific populations or under specific conditions.

**Why the Correct Answer is Right**
Some special incidence rates include:
- **Incidence rate** itself, which is the number of new cases per unit population at risk over a specified period.
- **Attack rate**, which is the number of new cases among exposed individuals over a specified period.
- **Incidence density**, which is the number of new cases per unit time at risk.
- **Cumulative incidence**, which is the number of new cases over a specified period among a cohort or population at risk.
However, one of the given options is not a special incidence rate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is a special incidence rate as it is used to describe the number of new cases among exposed individuals over a specified period.
**Option B:** This option is also a special incidence rate as it is used to describe the number of new cases per unit time at risk.
**Option C:** This option is another special incidence rate as it is used to describe the number of new cases over a specified period among a cohort or population at risk.
**Option D:** This option is not a special incidence rate as it is actually the term for the number of new cases per unit population at risk over a specified period, which is already referred to as the incidence rate.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that special incidence rates help in understanding the risk of disease development in different populations and under various conditions, and each type of incidence rate has its unique application in epidemiological studies.

**Correct Answer:** D. Incidence rate.