**Core Concept**
The question is testing the student's knowledge of pharmacotherapy for Parkinson's disease, specifically the role of catechol-O-methyltransferase (COMT) inhibitors in managing the condition. COMT inhibitors are a class of medications that work by blocking the enzyme COMT, which is involved in the breakdown of dopamine in the brain.

**Why the Correct Answer is Right**
The correct answer is a selective COMT inhibitor, which means it specifically targets the COMT enzyme to prevent the breakdown of dopamine. This allows dopamine to accumulate in the brain, thereby alleviating symptoms of Parkinson's disease. COMT inhibitors are often used in conjunction with levodopa to enhance its effectiveness and reduce the risk of levodopa-induced dyskinesias.

**Option A:** Entacapone - This is the correct answer. Entacapone is a selective, reversible COMT inhibitor that is commonly used in the treatment of Parkinson's disease.

**Option B:** Selegiline - Selegiline is a monoamine oxidase B (MAO-B) inhibitor, not a COMT inhibitor. It works by preventing the breakdown of dopamine by MAO-B, but it does not affect COMT activity.

**Option C:** Tolcapone - Tolcapone is another COMT inhibitor, but it is not selective and can have off-target effects. It is not the best choice as a selective COMT inhibitor.

**Option D:** Rasagiline - Rasagiline is a MAO-B inhibitor, similar to selegiline. It is not a COMT inhibitor and does not work by blocking the COMT enzyme.

**Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that COMT inhibitors can increase the risk of hypertension and cardiac events, particularly when used in combination with levodopa. Therefore, patients taking COMT inhibitors should be closely monitored for these potential side effects.

**Correct Answer:** A. Entacapone