Which of the following drugs is an opioid?

• A. Nitrous oxide
• B. Ketamine
• C. Pancuronium
• D. Morphine

Answer: (D)

Opioids act by binding to opioid receptors in the brain and spinal cord to produce analgesia, with possible sedation and respiratory depression. Morphine is a prototypical opioid that strongly activates mu receptors, providing effective pain relief in animals and people. The other drugs work through different mechanisms: nitrous oxide is an inhaled anesthetic/analgesic that does not act on opioid receptors; ketamine is a dissociative anesthetic that blocks NMDA receptors; and pancuronium is a neuromuscular blocker that paralyzes muscles by blocking acetylcholine at the neuromuscular junction. So the drug that fits the opioid category is morphine.