Which agent is a phenanthrene opioid that exhibits a full and immediate response to naloxone in case of overdose?

The correct answer is morphine, as it is a full agonist opioid from the phenanthrene class. In the case of an overdose, morphine's effects on the central nervous system are reversible with naloxone, which is an opioid antagonist. Naloxone works by competitively inhibiting the opioid receptors, thereby quickly reversing the life-threatening respiratory depression and sedation caused by morphine overdose.

Morphine's structure, pharmacodynamics, and full agonist properties contribute to its immediate response to naloxone when addressing overdose situations. Being a strong agonist means that it binds effectively to the mu-opioid receptors, leading to significant analgesic and euphoric effects, which can become dangerous in overdose scenarios. Naloxone's ability to displace morphine from these receptors allows for rapid recovery of respiratory function in individuals experiencing an opioid overdose.

In contrast, the other options represent either partial agonists, mixed agonist-antagonists, or substances that do not exhibit the same immediate response in overdose situations. This highlights the unique interaction that full agonist opioids like morphine have with naloxone in emergency treatment.