**Core Concept**
A Phase I blocker is a type of beta-blocker that selectively blocks the beta-1 receptors in the heart, reducing heart rate and contractility. This class of drugs is often used to manage hypertension, angina, and certain arrhythmias. Beta-1 receptors are primarily located in the heart, whereas beta-2 receptors are found in the lungs, blood vessels, and smooth muscle.

**Why the Correct Answer is Right**
The correct answer is a beta-1 selective adrenergic receptor blocker, such as atenolol or metoprolol. These drugs work by competitively inhibiting the binding of epinephrine and norepinephrine to beta-1 receptors, resulting in decreased heart rate and contractility. This action reduces the heart's oxygen demand, which is beneficial for patients with angina or heart failure.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because non-selective beta-blockers, such as propranolol, block both beta-1 and beta-2 receptors, leading to unwanted side effects like bronchoconstriction and peripheral vasoconstriction.
* **Option B:** is incorrect because alpha-1 blockers, such as prazosin, selectively block alpha-1 adrenergic receptors in the smooth muscle of blood vessels, causing vasodilation and hypotension.
* **Option C:** is incorrect because calcium channel blockers, such as verapamil, inhibit the influx of calcium ions into cardiac and smooth muscle cells, reducing heart rate and contractility, but through a different mechanism than beta-1 blockers.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that beta-1 blockers are contraindicated in patients with bronchospastic disease or asthma, as they can trigger bronchoconstriction by blocking beta-2 receptors in the lungs.

**Correct Answer:** A. Atenolol.