Which of the following is an example of an antiarrhythmic medication?

The correct answer identifies Lanoxin (Digoxin) as an antiarrhythmic medication. Digoxin is primarily used to treat certain types of heart rhythm disorders, most notably atrial fibrillation and atrial flutter. It works by slowing down the heart rate and helping to strengthen the heart's contractions, which can be crucial in managing arrhythmias.

In the context of the other medications listed, atenolol is a beta-blocker that can also be used for cardiac issues such as hypertension and certain arrhythmias, but it is not classified as a primary antiarrhythmic agent. Amoxicillin is an antibiotic used to treat bacterial infections, unrelated to heart rhythm management. Furosemide is a diuretic medication primarily used to eliminate excess fluid from the body, often in cases of heart failure or hypertension, but again, does not have a direct role in treating arrhythmias.

Thus, among the options, Digoxin stands out as an established antiarrhythmic medication due to its specific action on heart rhythm abnormalities.