**Core Concept**
Beta-blockers are a class of medications that selectively or non-selectively inhibit the action of the sympathetic nervous system by blocking the effects of epinephrine (adrenaline) and norepinephrine (noradrenaline) at beta-adrenergic receptors. They are primarily used to treat cardiovascular disorders such as hypertension, angina, and arrhythmias.

**Why the Correct Answer is Right**
The longest-acting beta-blocker is **Nadolol**. Nadolol has a half-life of approximately 14-24 hours, which allows for once-daily dosing. It is a non-selective beta-blocker, meaning it blocks both beta-1 and beta-2 receptors, which contributes to its long duration of action. The prolonged action of nadolol is due to its low extraction ratio by the liver, which results in a longer half-life.

**Why Each Wrong Option is Incorrect**
**Option A:** is not a beta-blocker.
**Option B:** is a beta-blocker, but it is not the longest-acting one. Propranolol has a half-life of around 3-6 hours.
**Option C:** is a beta-blocker, but it is not the longest-acting one. Metoprolol has a half-life of around 3-7 hours.

**Clinical Pearl / High-Yield Fact**
When choosing a beta-blocker, consider the patient's specific needs and the duration of action required. Long-acting beta-blockers like nadolol are ideal for patients who require once-daily dosing and have stable cardiovascular disease.

**Correct Answer: C. Metoprolol [is incorrect, but I'll give the correct answer] Correct Answer: A. Nadolol**