Final answer:
The opioid not approved for epidural or intrathecal use is remifentanil. While other opioids like fentanyl, sufentanil, and morphine are utilized in these routes, remifentanil's pharmacokinetic properties confine its use to intravenous administration in the context of general anesthesia.
Step-by-step explanation:
Remifentanil is the opioid that is currently not approved for epidural or intrathecal use, making d. remifentanil the correct answer. Opioids like fentanyl, sufentanil, and morphine are commonly utilized for their analgesic properties in various medical settings, including epidural and intrathecal administration to reduce severe pain.
Opioids work by binding to opioid receptors in the brain, spinal cord, and gastrointestinal tract, altering the perception of pain. Opioids such as codeine, oxycodone, and hydrocodone are prescribed for mild to severe pain but have a high potential for abuse.
While medications like morphine and fentanyl are approved for epidural and intrathecal use in clinical settings, remifentanil's rapid onset and short duration of action make it less suitable for these particular routes of administration. Therefore, its use is mostly confined to general anesthesia administered intravenously.