**Core Concept**
L-Dopa is a precursor to dopamine, which is used to treat Parkinson's disease. However, L-Dopa undergoes peripheral decarboxylation to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) before it reaches the central nervous system (CNS). This peripheral conversion reduces the amount of L-Dopa that crosses the blood-brain barrier, thereby decreasing its effectiveness in increasing CNS dopamine levels.

**Why the Correct Answer is Right**
Carbidopa is a peripheral decarboxylase inhibitor that selectively inhibits the enzyme AADC outside the CNS. By doing so, carbidopa prevents the peripheral conversion of L-Dopa to dopamine, thereby increasing the amount of L-Dopa that crosses the blood-brain barrier and enters the CNS. This allows more L-Dopa to be available for conversion to dopamine within the CNS, thereby increasing its effectiveness in treating Parkinson's disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because carbidopa does not decrease the efficacy of L-Dopa; rather, it increases its effectiveness by preventing peripheral decarboxylation.
**Option C:** This option is incorrect because carbidopa does not increase the dose of L-Dopa required; rather, it allows a lower dose of L-Dopa to be effective due to increased availability of L-Dopa in the CNS.
**Option D:** This option is incorrect because carbidopa actually promotes the conversion of L-Dopa to dopamine in the CNS by preventing peripheral decarboxylation, rather than inhibiting it.

**Clinical Pearl / High-Yield Fact**
Carbidopa-L-Dopa combinations are often used to treat Parkinson's disease because they allow for a more efficient and effective delivery of L-Dopa to the CNS, thereby reducing the required dose and associated side effects.

**✓ Correct Answer: B. Decrease the efficacy of levodopa**