**Core Concept**
Levodopa is a precursor to dopamine, used in the treatment of Parkinson's disease. Its therapeutic effect relies on its conversion to dopamine by the enzyme dopa decarboxylase, which is present in both the central nervous system (CNS) and peripheral tissues.

**Why the Correct Answer is Right**
The correct answer is an enzyme inhibitor that blocks the action of dopa decarboxylase, thereby preventing the conversion of levodopa to dopamine. This results in the depletion of dopamine in the CNS, effectively abolishing the therapeutic effect of levodopa. The enzyme in question is **carbidopa**, which is often co-administered with levodopa to prevent peripheral conversion and increase the bioavailability of levodopa in the CNS.

**Why Each Wrong Option is Incorrect**
**Option A:** **MAO-B inhibitors** do not affect the conversion of levodopa to dopamine, but rather inhibit the breakdown of dopamine by monoamine oxidase B (MAO-B), thus increasing its availability.

**Option B:** **COMT inhibitors** prevent the breakdown of levodopa by catechol-O-methyltransferase (COMT), thereby increasing its availability in the CNS.

**Option C:** **Serotonin syndrome** is a potential side effect of levodopa, but it is not related to the abolition of its therapeutic effect.

**Clinical Pearl / High-Yield Fact**
Carbidopa, a peripheral decarboxylase inhibitor, is essential in the management of Parkinson's disease, as it significantly increases the bioavailability of levodopa in the CNS.

**Correct Answer:** B. COMT inhibitors.