All are opioid agonist – antagonist compounds except – a) Buprenorphineb) Nalbuphinec) Pentazocined) Nalmefenee) Papaverine

**Core Concept**
The question tests the understanding of opioid agonist-antagonist compounds, which are drugs that have both agonist and antagonist effects on opioid receptors. These compounds are used for their analgesic properties while having a lower risk of addiction compared to pure opioid agonists.

**Why the Correct Answer is Right**
Papaverine is a non-narcotic opioid derivative that acts as a smooth muscle relaxant and does not exhibit opioid agonist-antagonist properties. Buprenorphine, Nalbuphine, and Pentazocine are known opioid agonist-antagonist compounds, and Nalmefene is an opioid antagonist.

**Why Each Wrong Option is Incorrect**
**Option A:** Buprenorphine is an opioid agonist-antagonist, acting as a partial agonist at the mu receptor and an antagonist at the kappa receptor.
**Option B:** Nalbuphine is an opioid agonist-antagonist, used for its analgesic effects with less risk of respiratory depression.
**Option C:** Pentazocine is an opioid agonist-antagonist, acting as an agonist at kappa receptors and an antagonist or partial agonist at mu receptors.
**Option D:** Nalmefene is actually an opioid antagonist, used for the treatment of alcohol dependence, but it's not an agonist-antagonist.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that while opioid agonist-antagonists have a lower risk of addiction, they can still cause significant side effects and should be used judiciously.

**Correct Answer:** e) Papaverine