**Core Concept**
Naltrexone is an opioid receptor antagonist that is used in the management of opioid dependence and alcohol dependence. It works by blocking the opioid receptors in the brain, thereby preventing the euphoric effects of opioids and reducing the craving for alcohol.

**Why the Correct Answer is Right**
Naltrexone is a competitive antagonist at the mu opioid receptor, which is the primary receptor responsible for the euphoric effects of opioids. By blocking the mu opioid receptor, naltrexone reduces the reinforcing effects of opioids and decreases the risk of relapse in individuals with opioid dependence. Naltrexone also has a role in managing alcohol dependence, as it is thought to reduce the craving for alcohol by blocking the release of dopamine in the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** Naltrexone is not a partial agonist at the mu opioid receptor. Instead, it is a competitive antagonist that completely blocks the receptor.
**Option B:** Naltrexone is not a long-acting opioid agonist. It is actually a long-acting opioid antagonist that can block opioid receptors for up to 30 days.
**Option C:** Naltrexone is not a medication used to treat acute opioid overdose. Instead, it is used to prevent relapse in individuals with opioid dependence.

**Clinical Pearl / High-Yield Fact**
Naltrexone should be used with caution in individuals with a history of opioid dependence, as it may precipitate withdrawal symptoms if used too soon after last opioid use.

**Correct Answer: D. Naltrexone is not a medication used to treat acute opioid overdose.**