**Core Concept**
The opioid analgesics primarily act through various opioid receptors, including mu (μ), kappa (κ), and delta (δ) receptors. The kappa opioid receptors are involved in analgesia, but their activation also leads to dysphoria and psychotomimetic effects.

**Why the Correct Answer is Right**
The correct answer is related to the kappa opioid receptors, which are activated by certain opioid analgesics. These receptors are located in the brain and spinal cord. Activation of kappa opioid receptors results in analgesia, but it also causes psychotomimetic effects such as dysphoria and hallucinations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a specific opioid analgesic that acts primarily through kappa opioid receptors.
**Option B:** This option is incorrect because it is a mu opioid receptor agonist, not a kappa opioid receptor agonist.
**Option C:** This option is incorrect because it is not a primary kappa opioid receptor agonist.

**Clinical Pearl / High-Yield Fact**
One important clinical fact is that kappa opioid receptor agonists can cause psychotomimetic effects, which can be a problem in patients with a history of psychiatric disorders. Therefore, these agents should be used with caution in such patients.

**Correct Answer: C. Tramadol**