Which drug is used to treat certain cardiac arrhythmias?

• A. Vaccines
• B. Magnesium sulfate
• C. Adenosine
• D. Cimetidine

Answer: (C)

Adenosine works by briefly blocking conduction through the AV node, which interrupts reentrant circuits that cause certain supraventricular tachycardias. This makes it the drug of choice for acute termination of paroxysmal SVT because of its very short duration of action—only a few seconds—so a rapid IV push can reset the rhythm and return a normal heartbeat. The effect is so brief that the patient may feel a transient pause in rhythm, a flush, or chest discomfort, but it rapidly wears off. This pharmacologic profile is ideal for AV nodal–dependent tachycardias and explains why adenosine is used in these cases. It’s not effective for atrial fibrillation with rapid response or ventricular tachycardia, and the other listed items don’t target AV nodal conduction in this way: vaccines have no antiarrhythmic effect, cimetidine is a gastric medication, and magnesium sulfate is reserved for specific arrhythmias such as torsades de pointes or certain electrolyte-related issues.