This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S, Subject name: Pharmacology
In treatment of parkinsonism, levodopa is combined with carbidopa: 
Select and explain. 
A. Decrease the treatment duration, B. Decrease the central side effects of L-dopa, C. Decrease the effectiveness of L -Dopa, D. Increase the crossing of levodopa through blood brain barrier
D: Increase the crossing of levodopa through blood brain barrier
Answer: D: Increase the crossing of levodopa through blood brain barrier 
Explanation: 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.