**Core Concept**
Fentanyl is a potent opioid analgesic used for pain management, anesthesia, and palliative care. It works by activating mu opioid receptors in the central nervous system, leading to analgesia and euphoria. However, its use is associated with several side effects due to its potent opioid activity.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of action of fentanyl, which involves the activation of mu opioid receptors. This activation can lead to a range of side effects, including respiratory depression, sedation, and constipation. One of the common side effects of fentanyl is **respiratory depression**, which occurs due to the decrease in respiratory rate and depth caused by the activation of mu opioid receptors in the brainstem. This can be life-threatening in severe cases and requires close monitoring.

**Why Each Wrong Option is Incorrect**
* **Option A:** Nausea and vomiting are common side effects of opioids, but they are not the most characteristic side effect of fentanyl.
* **Option B:** Pruritus (itching) can occur with opioids, but it is not as common with fentanyl as respiratory depression.
* **Option C:** Hypertension is not typically associated with fentanyl, as opioids usually cause vasodilation and decreased blood pressure.

**Clinical Pearl / High-Yield Fact**
Fentanyl and other opioids can cause respiratory depression even at therapeutic doses, and their use should be closely monitored, especially in patients with pre-existing respiratory disease or those taking other central nervous system depressants.

**Correct Answer: C. Hypertension**