A patient who is taking morphine for pain asks the nurse how a pain medication can also cause constipation. What does the nurse know about morphine?

Choice A reason: Morphine, an opioid, binds to mu, kappa, and delta receptors in the brain, spinal cord, and gastrointestinal tract. Mu receptors in the gut slow peristalsis, causing constipation, while central receptors relieve pain. This multi-receptor binding explains both therapeutic and side effects, making this the correct choice. Choice B reason: Constipation from morphine occurs at therapeutic, not just toxic, doses due to mu receptor activation in the gastrointestinal tract, which reduces motility. Toxicity may worsen side effects, but constipation is a common effect at standard doses, making this choice inaccurate for explaining morphine’s mechanism. Choice C reason: Morphine’s receptors (e.g., mu) regulate multiple processes, but the drug itself isn’t selective to multifunctional receptors. It binds broadly to opioid receptors, causing both analgesia and side effects like constipation. This choice misrepresents morphine’s non-selective binding, making it less accurate than choice A. Choice D reason: Morphine’s effects, including analgesia and constipation, result from specific receptor binding, not coincidental processes. It activates opioid receptors in the brain for pain relief and in the gut for reduced motility. This choice incorrectly suggests constipation is unrelated to morphine’s pharmacological action, making it incorrect.