## Core Concept
Naltrexone is a medication used primarily in the management of opioid dependence and alcohol dependence. It belongs to a class of drugs known as opioid antagonists, which work by blocking the opioid receptors in the brain. This action prevents the euphoric effects of opioids and reduces cravings.

## Why the Correct Answer is Right
Naltrexone is an opioid receptor antagonist. It works by competitively binding to opioid receptors, thereby blocking the action of endogenous opioids and exogenously administered opioids. This mechanism makes it useful for treating opioid addiction and for managing alcohol dependence, as it can help reduce the craving for alcohol.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately describe naltrexone.
- **Option B:** This option is incorrect as it does not correctly identify naltrexone's pharmacological class or its use.
- **Option C:** This option might seem plausible but is incorrect because naltrexone is specifically an antagonist, not an agonist or a partial agonist.

## Clinical Pearl / High-Yield Fact
A key point to remember about naltrexone is that it can be used for the treatment of alcohol dependence. It's also important to note that before starting naltrexone for opioid dependence, the patient must be opioid-free for a certain period (usually 7-10 days) to prevent severe withdrawal symptoms.

## Correct Answer: D. Opioid receptor antagonist.