Naloxone's mechanism of action is:

Correct answer: D. Competitive mu-opioid receptor ANTAGONIST (reverses opioid toxicity)

• A. Mu-opioid receptor agonists (morphine, oxycodone, fentanyl) cause opioid effects — the opposite of naloxone.
• B. NMDA antagonism is the mechanism of ketamine and memantine — not naloxone.
• C. GABA-A antagonism (flumazenil reverses benzodiazepines, not opioids) — wrong receptor system.
• D. Correct. Naloxone (Narcan) is a PURE competitive mu-opioid receptor ANTAGONIST. It has higher affinity for mu receptors than opioids, rapidly displacing them and reversing opioid-induced: respiratory depression, sedation, miosis, and analgesia. Duration of action is 30–90 minutes (shorter than most opioids), requiring repeat dosing or infusion for long-acting opioid overdoses.