Which of the following medications is classified as a beta-adrenergic blocker?

Propranolol is classified as a beta-adrenergic blocker, which means it primarily works by blocking the action of certain neurotransmitters, such as adrenaline, on beta-adrenergic receptors. This action leads to a decrease in heart rate, reduced force of heart contractions, and less oxygen demand by the heart muscle. It is commonly used to manage conditions such as hypertension, anxiety, and certain types of arrhythmias.

Understanding the other medications provides insight into why they do not fit this classification. Nitroglycerin, for instance, is a nitrate that works by dilating blood vessels, thus decreasing chest pain but does not interact with beta-adrenergic receptors. Isosorbide mononitrate is also a nitrate used for angina, and it operates through a similar mechanism as nitroglycerin. Amlodipine is a calcium channel blocker, which affects calcium entry into the cells of the heart and blood vessels, leading to vasodilation and a reduction in blood pressure, but it does not block beta-adrenergic receptors.

Thus, Propranolol stands out as the correct answer due to its specific action on beta-adrenergic receptors, distinguishing it from the other listed medications.