In treatment of parkinsonism, levodopa is combined with carbidopa:

Ans. D. Increase the crossing of levodopa through blood brain barrierLevodopa is drug given for parkinsonism. It is metabolized by Dopa decarboxylase enzyme which is present peripherally. So when given alone it get metabolized and do not cross the blood brain barrier. Carbidopa on other hand is Dopa decarboxylase inhibitor which protect levodopa and allows its entry into the brain.