**Core Concept**
Parkinson's disease is a neurodegenerative disorder characterized by the loss of dopamine-producing neurons in the substantia nigra. Dopamine replacement therapy is a cornerstone of treatment, but its efficacy is limited by the peripheral conversion of levodopa to dopamine by the enzyme catechol-O-methyltransferase (COMT). Selective COMT inhibitors can increase the bioavailability of levodopa by preventing this peripheral conversion.

**Why the Correct Answer is Right**
Entacapone is a selective, reversible inhibitor of COMT. By selectively inhibiting COMT, entacapone increases the plasma concentrations of levodopa, thereby enhancing its delivery to the brain and prolonging its therapeutic effects. This is particularly beneficial in patients with Parkinson's disease who are on levodopa/carbidopa therapy, as it allows for more flexible dosing regimens and improved symptom control.

**Why Each Wrong Option is Incorrect**
**Option A:** Selegiline is an irreversible monoamine oxidase B (MAO-B) inhibitor, which is used in the treatment of Parkinson's disease but is not a COMT inhibitor.

**Option B:** Tolcapone is another selective COMT inhibitor, but it is not as commonly used as entacapone due to concerns about its potential hepatotoxicity.

**Option C:** Rasagiline is a selective, irreversible MAO-B inhibitor, similar to selegiline, and is not a COMT inhibitor.

**Option D:** Dopamine agonists, such as pramipexole and ropinirole, are used to treat Parkinson's disease but do not inhibit COMT.

**Clinical Pearl / High-Yield Fact**
Selective COMT inhibitors like entacapone are often used in combination with levodopa/carbidopa to enhance the bioavailability of levodopa and prolong its therapeutic effects.

**Correct Answer:** B. Tolcapone