Which of the following is a key reason for using carbidopa with levodopa?

The combination of carbidopa with levodopa is essential in the treatment of Parkinson's disease primarily because carbidopa reduces the peripheral side effects associated with levodopa. Levodopa is a precursor to dopamine and is converted to dopamine in both the central nervous system (CNS) and the peripheral tissues. However, the conversion of levodopa to dopamine in the periphery can lead to side effects such as nausea, vomiting, and cardiovascular issues.

By administering carbidopa alongside levodopa, the conversion of levodopa to dopamine in the peripheral system is inhibited; carbidopa prevents the enzyme dopa decarboxylase from acting outside of the CNS. This means that more levodopa is available to cross the blood-brain barrier, where it can then be converted to dopamine, effectively increasing its therapeutic efficacy while minimizing undesirable peripheral effects. This combination is therefore crucial for enhancing the quality of life for individuals with Parkinson's disease, as it allows for lower doses of levodopa to be used without compromising treatment outcomes.