MAT is identified by irregular atrial rhythm with multiple atrial foci firing, which produces at least three distinct P-wave morphologies and varying PR intervals from beat to beat. The rhythm is tachycardic, so the heart rate is typically over 100 beats per minute. Delta waves, on the other hand, are a sign of pre-excitation from an accessory pathway as seen in Wolff-Parkinson-White syndrome and are not features of MAT. So the presence of a delta wave would point away from MAT and toward WPW instead.

These medications are loop diuretics, indicated by the -semide suffix. They act in the thick ascending limb of the loop of Henle by inhibiting the Na+-K+-2Cl- cotransporter, which prevents reabsorption of sodium, potassium, and chloride. Water follows, producing a strong diuretic effect. Because of their potency, they’re especially useful for removing excess fluid in conditions like heart failure with edema, pulmonary edema, or certain kidney diseases. They work faster and cause more diuresis than other diuretic classes. Common examples include furosemide, torsemide, and bumetanide. Be mindful of electrolyte and fluid balance: they can cause hypokalemia, hypomagnesemia, dehydration, and increased uric acid levels, which can trigger gout. Ototoxicity is a potential risk, especially with rapid IV administration. In contrast, thiazide diuretics act on the distal tubule and are less potent; potassium-sparing diuretics act on the collecting duct and tend to preserve potassium; osmotic diuretics work by creating an osmotic gradient rather than blocking transporters and are used for different indications.

Class II antiarrhythmic drugs are beta-adrenergic blockers. In the cardiac nodal tissues (SA and AV nodes), the pace of automaticity is set by the slope of phase 4. Beta-blockade reduces sympathetic stimulation, lowers cAMP, and decreases calcium entry, which together slow the rate of spontaneous depolarization. This depresses the slope of phase 4 and slows nodal firing, leading to a slower nodal action potential. It also slows AV nodal conduction, which is the hallmark effect of these drugs. The other options don’t fit this mechanism. Lengthening phase 0 and the effective refractory period is not the primary action of beta-blockers (that’s more characteristic of other classes that alter repolarization). Increasing the depolarization threshold isn’t the typical direct effect of Class II agents, and outright blocking calcium channels describes calcium channel blockers (Class IV), not beta-blockers.

Class III antiarrhythmic effects come from blocking potassium channels responsible for the heart’s repolarization. By inhibiting this outward K+ current during phase 3, repolarization is slowed, so the action potential lasts longer. That lengthened action potential duration also prolongs the effective refractory period, making the tissue less capable of re-exciting and helping prevent re-entrant arrhythmias. These drugs do not lengthen the upstroke phase 0 (driven by sodium channels) or raise the depolarization threshold, which would be more characteristic of other drug effects. Keep in mind the trade-off: QT prolongation, with a risk of torsades de pointes in some settings.