Final answer:
Amantadine is used to treat Parkinson's disease by impacting dopamine and NMDA in the brain, leading to a therapeutic decrease in tremors.
Step-by-step explanation:
The question refers to the therapeutic effect of amantadine, a medication with antiviral properties that is also used to treat Parkinson's disease. When amantadine is effective, it impacts brain chemicals such as dopamine and NMDA, which in turn can reduce symptoms related to Parkinson's disease. Specifically, a client who has been taking amantadine might experience a decrease in tremors, which are a common symptom of Parkinson's disease.