Buprenorphine paial agonist at which opioid receptor?

**Core Concept**
Buprenorphine is a medication used to treat opioid use disorder and pain, acting as a partial agonist at specific opioid receptors. The **mu (μ) opioid receptor** is a key target for many opioid medications.

**Why the Correct Answer is Right**
Buprenorphine's partial agonist activity is primarily at the **mu (μ) opioid receptor**, which is responsible for its therapeutic effects, including analgesia and reduction of opioid withdrawal symptoms. This action at the **mu (μ) receptor** also contributes to its ceiling effect on respiratory depression, making it a safer option compared to full agonists.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while buprenorphine does have some affinity for other opioid receptors, its primary and most clinically relevant action as a partial agonist is not at this receptor.
**Option C:** Incorrect as buprenorphine's action is more closely associated with mu receptors rather than kappa receptors for its primary therapeutic effects.
**Option D:** Incorrect because buprenorphine's mechanism does not primarily involve delta opioid receptors for its main clinical use.

**Clinical Pearl / High-Yield Fact**
Remember, buprenorphine's partial agonism at the **mu (μ) opioid receptor** is crucial for its role in managing opioid dependence, as it helps mitigate withdrawal symptoms without causing significant euphoria or dangerous side effects like full mu receptor agonists.

**Correct Answer:** D. mu.