Final answer:
The change from plain levodopa to levodopa-carbidopa (Sinemet) helps to increase the effectiveness of the treatment for Parkinson's disease by allowing more levodopa to reach the brain, reducing the required dosage, and minimizing side effects.
Step-by-step explanation:
The best rationale for changing from 'plain levodopa' to levodopa-carbidopa (Sinemet) for a patient with Parkinson's disease is to enhance the effectiveness of the levodopa therapy and reduce side effects. Carbidopa is added to inhibit the enzyme that breaks down levodopa before it can reach the brain, allowing for more efficient transport across the blood-brain barrier and reducing the amount of levodopa required. This also helps to lessen peripheral side effects. Additionally, carbidopa does not cross the blood-brain barrier, which means it exclusively targets the enzyme outside the brain, optimizing the amount of levodopa that gets converted into dopamine within the brain itself, where it's needed to help manage Parkinson's symptoms. Over time, patients tend to become less responsive to levodopa treatment, and this combination aims to combat that tolerance and improve symptom control.