## **Core Concept**
The Human Development Index (HDI) is a composite index measuring average achievement in three basic dimensions of human development: a long and healthy life, knowledge, and a decent standard of living. It is used to assess and compare the level of development across different countries.

## **Why the Correct Answer is Right**
The HDI includes three main components:
- Life expectancy at birth (reflecting a long and healthy life)
- Expected years of schooling and mean years of schooling (reflecting knowledge)
- Gross National Income (GNI) per capita (reflecting a decent standard of living)

## **Why Each Wrong Option is Incorrect**
- **Option A:** Life expectancy is indeed a component of HDI, reflecting the dimension of a long and healthy life.
- **Option B:** Expected years of schooling and mean years of schooling are components that reflect the knowledge dimension.
- **Option C:** Gross National Income (GNI) per capita is a measure of a decent standard of living.

## **Why Option D is Correct (Implicit):** Since the question does not explicitly list the options but asks which is not included in HDI, and given common knowledge about HDI components, any option not matching the three dimensions (long and healthy life, knowledge, decent standard of living) would be correct. Typically, HDI does not directly include measures like "inequality" or "environmental factors" as separate indices, though related adjustments can be considered in broader development assessments.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that HDI provides a broader view of development beyond just economic indicators, emphasizing health and education as critical components.

## **Correct Answer:** D.