A client has just received a high-dose fentanyl infusion for severe pain management. The nurse should monitor the client most closely for which adverse effect?

Rationale: A. Opioids like fentanyl are not typically associated with increased urination or incontinence. In fact, they may cause urinary retention rather than increased output. Therefore, this is incorrect. B. Fentanyl is a potent opioid analgesic that can significantly depress the central nervous system, particularly the respiratory center in the brain. High doses increase the risk of respiratory depression, which can lead to hypoventilation, hypoxia, and potentially death if not promptly recognized and managed. This is the most critical adverse effect to monitor, making this the correct answer. C. Opioids generally do not cause increased heart rate; they are more likely to cause bradycardia due to their depressant effects on the central nervous system. Therefore, this is incorrect. D. Fentanyl is more commonly associated with hypotension rather than elevated blood pressure due to vasodilation and decreased sympathetic tone. Therefore, this option is incorrect.