Final answer:
Trihexyphenidyl, used for Parkinson's disease, can cause urinary retention due to its anticholinergic properties, which decrease bladder muscle tone.
Step-by-step explanation:
A client taking trihexyphenidyl for the treatment of Parkinson's disease should be monitored for urinary retention as a side effect. Trihexyphenidyl is an anticholinergic agent which can lead to decreases in smooth muscle tone and inhibition of bladder contraction, hence causing urinary retention. It is less likely to cause sedation compared to other side effects, and it is not associated with hypertension or diarrhea in the context of Parkinson's treatment. Monitoring for urinary retention is essential because it can lead to urinary tract infections, bladder damage, and impact the quality of life.