Which drug is NOT a beta blocker?

Quinidine is the correct answer because it is an antiarrhythmic medication primarily used to treat certain types of irregular heartbeats. It falls under a different class of drugs called Class 1 antiarrhythmics, which work by stabilizing the cardiac cell membrane, thereby influencing the conduction and excitability of the cardiac tissues.

In contrast, propranolol, atenolol, and metoprolol are all beta blockers, which are a class of medications that specifically block the action of epinephrine and norepinephrine at beta-adrenergic receptors. This action helps to lower heart rate, reduce blood pressure, and alleviate stress on the heart, making beta blockers commonly prescribed for conditions such as hypertension, anxiety, and certain heart conditions. Understanding the distinct properties of these drug classes is crucial for effective management in clinical practice.