A nursing student asks the nurse why patients who have parkinsonism receive a combination of carbidopa and levodopa. The nurse will explain that the combination product
a. allows larger doses of levodopa to be given without causing increased adverse reactions.
b. causes more levodopa to be converted to dopamine before crossing the blood-brain barrier.
c. eliminates almost all drug side effects of both levodopa and carbidopa.
d. reduces peripheral side effects by inhibiting decarboxylase in the peripheral nervous system.
ANS: D
Without carbidopa, about 99% of levodopa is converted to dopamine before crossing the blood-brain barrier, causing peripheral adverse effects. When carbidopa is added, the enzyme decarboxylase is inhibited, allowing levodopa to cross into the brain before being converted to the active metabolite dopamine. The result is less levodopa required to achieve the desired effect. The drug still has many side effects, but the peripheral effects are lessened.
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