Final answer:
Levodopa-carbidopa is prescribed for Parkinson's disease to increase dopamine in the brain. Levodopa converts to dopamine, while carbidopa prevents its early conversion, ensuring more reaches the brain.
Step-by-step explanation:
The levodopa-carbidopa combination is used in the treatment of Parkinson's disease because these two drugs work together to effectively increase dopamine levels in the brain. Levodopa is a precursor to dopamine that can cross the blood-brain barrier, and once in the brain, it is converted to dopamine, which helps to compensate for the decreased dopaminergic neuron activity in the substantia nigra. However, levodopa can be broken down in the bloodstream before it reaches the brain, leading to less efficacy and more side effects. Carbidopa acts to inhibit the enzyme aromatic L-amino acid decarboxylase in the periphery, which means that more levodopa is available to enter the brain and be converted to dopamine where it is needed.
Thus, the nurse's best response would be: 'The pill contains two medications; levodopa helps to replace dopamine, a neurotransmitter that's deficient in Parkinson's disease, and carbidopa prevents levodopa from being converted to dopamine outside the brain, allowing more to be available where it's needed.'