**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 mu-opioid receptors, thereby reducing the euphoric effects of opioids and preventing relapse.

**Why the Correct Answer is Right**
Naltrexone acts by binding to opioid receptors in the brain, specifically the mu-opioid receptors, which are involved in the perception of pain and the euphoric effects of opioids. By blocking these receptors, naltrexone prevents the release of dopamine, a neurotransmitter associated with pleasure and reward. This leads to a decrease in the desire to use opioids and a reduction in the risk of relapse.

**Why Each Wrong Option is Incorrect**
**Option A:** Naltrexone is not a partial agonist at opioid receptors, it is a full antagonist.
**Option B:** Naltrexone does not have a significant effect on the mu-opioid receptors in the gastrointestinal tract, which is why it does not cause significant constipation like other opioid antagonists.
**Option C:** Naltrexone is not used as a first-line treatment for acute opioid overdose, naloxone is the preferred choice due to its rapid onset of action.

**Clinical Pearl / High-Yield Fact**
Naltrexone can precipitate opioid withdrawal in individuals who are physically dependent on opioids, which is why it should be started after a period of opioid abstinence.

**Correct Answer: D. Naltrexone is not used as a first-line treatment for acute opioid overdose, naloxone is the preferred choice due to its rapid onset of action.**