Final answer:
The therapeutic effect of baclofen for a client with multiple sclerosis is indicated by decreased muscle spasticity.
Step-by-step explanation:
The nurse can determine the therapeutic effect of baclofen in a client with multiple sclerosis by noting a decrease in muscle spasticity. Baclofen is a muscle relaxant and its effectiveness is majorly characterized by its ability to reduce excessive muscle contraction and resistance to stretch, often seen in conditions like multiple sclerosis that may result in spasticity and hyperflexia. Hence, when the medication is working, the client should experience less muscle stiffness and a reduction in involuntary muscle contractions.