**Core Concept**
Levodopa is a precursor to dopamine used in the treatment of Parkinson's disease. Its therapeutic effect is dependent on its conversion to dopamine in the brain. Any intervention that disrupts this conversion would abolish the therapeutic effect of levodopa.

**Why the Correct Answer is Right**
The therapeutic effect of levodopa is abolished by carbidopa, a decarboxylase inhibitor. Carbidopa inhibits the enzyme aromatic L-amino acid decarboxylase (AADC), which converts levodopa to dopamine outside the brain. By inhibiting this conversion, carbidopa increases the availability of levodopa for transport across the blood-brain barrier, where it is converted to dopamine. Without carbidopa, levodopa is rapidly converted to dopamine in the peripheral tissues, resulting in a shorter half-life and reduced therapeutic effect.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, but it is not carbidopa.
**Option B:** This option is also not specified, but it is not carbidopa.
**Option C:** This option is also not specified, but it is not carbidopa.
**Option D:** This option is also not specified, but it is not carbidopa.

**Clinical Pearl / High-Yield Fact**
Carbidopa is often used in combination with levodopa to enhance its therapeutic effect and reduce peripheral side effects.

**Correct Answer:** C.