Which of the following is an opioid analgesic?

• A. Morphine
• B. Acetaminophen
• C. Ibuprofen
• D. Diphenhydramine

Answer: (A)

Opioid analgesics work by binding to opioid receptors in the central nervous system to block pain signals and alter the perception of pain. Morphine is the classic opioid analgesic because it directly activates these receptors, particularly the mu receptors, producing strong analgesia and typical opioid effects such as relief of severe pain, along with potential respiratory depression and dependence with use. The other substances listed do not act as opioids: acetaminophen provides analgesia and fever relief through central mechanisms but not via opioid receptors; ibuprofen is an NSAID that reduces inflammation and pain by inhibiting COX enzymes; diphenhydramine is an antihistamine with sedative properties rather than an analgesic acting on opioid receptors. Therefore, morphine is the opioid analgesic.