## Core Concept
The infant mortality rate (IMR) is a measure of the number of deaths of infants under one year of age per 1000 live births in a given year. It is a critical indicator of the health and well-being of a population, reflecting the level of prenatal care, maternal health, and the effectiveness of the healthcare system.

## Why the Correct Answer is Right
The correct answer, stillbirth, is not included in the infant mortality rate because IMR specifically counts deaths of live-born infants. Stillbirths, or fetal deaths, occur when a fetus dies in the womb after a certain gestational age, usually 28 weeks, and are not considered part of the live births from which the IMR is calculated.

## Why Each Wrong Option is Incorrect
- **Option A:** Deaths due to congenital anomalies are indeed included in the infant mortality rate. These are conditions that a baby is born with and can significantly impact infant health.
- **Option B:** Deaths due to prematurity are also included in the IMR. Premature infants are at higher risk of death in the first year of life.
- **Option C:** Deaths due to infections are included as well. Infections can be a significant cause of morbidity and mortality in infants.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the infant mortality rate is sensitive to improvements in prenatal care, delivery practices, and neonatal care. High IMR values often reflect inadequate healthcare services, poor maternal health, and socioeconomic disparities.

## Correct Answer Line
**Correct Answer: D. stillbirth.**