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

**Why the Correct Answer is Right**
Naltrexone is a long-acting opioid antagonist that is administered orally or via injection. It has a high affinity for the mu-opioid receptor, which is the primary receptor involved in the euphoric effects of opioids. By blocking this receptor, naltrexone can help to reduce the risk of relapse in individuals with opioid dependence. Additionally, naltrexone has been shown to have a role in the treatment of alcohol dependence, as it can help to reduce the craving for alcohol and improve abstinence rates.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because naltrexone is not a partial agonist at the mu-opioid receptor. While it does have a high affinity for this receptor, its action is purely antagonistic, meaning that it blocks the effects of opioids rather than producing any agonist effects itself.

**Option B:** This option is incorrect because naltrexone is not a selective agonist at the kappa-opioid receptor. While it does have some affinity for this receptor, its primary action is at the mu-opioid receptor.

**Option C:** This option is incorrect because naltrexone is not metabolized by the liver to produce an active metabolite. Instead, it is primarily excreted unchanged in the urine.

**Option D:** This option is incorrect because naltrexone is not a direct agonist at the delta-opioid receptor. While it does have some affinity for this receptor, its primary action is at the mu-opioid receptor.

**Clinical Pearl / High-Yield Fact**
It's essential to note that naltrexone can induce withdrawal symptoms in individuals who have been taking opioids regularly, as it blocks the effects of opioids and can precipitate withdrawal. Therefore, it's crucial to taper off opioids before initiating naltrexone therapy.

**Correct Answer:** D.