**Core Concept**
The Poverty Index, also known as the Human Poverty Index (HPI), is a composite measure used to assess the level of deprivation in a population. It takes into account three dimensions: a long and unhealthy life, lack of knowledge, and a miserable standard of living.

**Why the Correct Answer is Right**
The HPI is calculated based on three key indicators: a low life expectancy at birth, a lack of access to education, and a low level of income. It does not include a measure of unemployment, as this is not a direct indicator of poverty. The HPI is designed to capture the multiple aspects of poverty, including health, education, and living standards.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the HPI does not specifically focus on income inequality, although it does take into account the level of income as one of its indicators.
**Option B:** This option is incorrect because the HPI does not include a direct measure of access to healthcare, although it does consider life expectancy at birth as one of its indicators.
**Option C:** This option is incorrect because the HPI does not specifically focus on environmental degradation, although it does take into account the standard of living as one of its indicators.

**Clinical Pearl / High-Yield Fact**
The HPI is a useful tool for policymakers and researchers to understand the complex nature of poverty and to identify areas where interventions can be made to improve the lives of the most vulnerable populations.

**Correct Answer: D.**