**Core Concept:** Opioid drugs are a class of medications that act on opioid receptors, which are G-protein coupled receptors, primarily located in the central nervous system and gastrointestinal tract. Agonists bind to and activate these receptors, while antagonists bind to these receptors but do not activate them, thereby blocking the effects of agonists and agonist-agonist mixtures.

**Why the Correct Answer is Right:** Morphine is a well-known opioid agonist that binds to and activates opioid receptors, resulting in analgesia, sedation, and respiratory depression. Naloxone is a specific opioid antagonist, binding to opioid receptors without activating them. When administered in combination with morphine, naloxone blocks the effects of morphine, effectively counteracting its side effects and restoring breathing to normal.

**Why Each Wrong Option is Incorrect:**

A. Naltrexone is another opioid antagonist, but it does not have the same potency as naloxone. In contrast to naloxone, it does not reverse the respiratory depression caused by morphine.

B. Intrathecal administration of naloxone would not block the respiratory depression caused by systemic morphine, as it does not cross the blood-brain barrier effectively.

C. Fentanyl is a potent opioid agonist, not an antagonist, and does not reverse the side effects of morphine when combined.

D. Methadone acts as both an agonist and antagonist, but in this context, it is considered an agonist as it potentiates morphine's effects, not counteract them.

**Clinical Pearl:** In clinical practice, naloxone is commonly used to rapidly reverse opioid overdose and counteract respiratory depression. However, when considering opioid agonist-antagonist combinations, it is essential to choose the appropriate antagonist for the specific opioid agonist being used, as different antagonists have varying potencies and modes of action.

**Correct Answer:** D. Methadone acts as both an agonist and antagonist, but in this context, it is considered an agonist as it potentiates morphine's effects, not counteract them.