B. Carbidopa: moa

Carbidopa, a dopa decarboxylase inhibitor, diminishes the metabolism of levodopa in the gastrointestinal tract and peripheral tissues, thereby increasing the availability of levodopa to the CNS. The addition of carbidopa lowers the dose of levodopa needed by four- to fivefold and, consequently, decreases the severity of the side effects arising from peripherally formed dopamine.