Which substrate is commonly involved in reentrant AVNRT?

• A. Dual AV nodal pathways
• B. Single AV node
• C. AV annulus only
• D. Purkinje fibers

Answer: (A)

Reentrant AVNRT relies on dual AV nodal physiology, with two distinct conduction pathways inside or near the AV node—the slow pathway and the fast pathway. A premature atrial beat can travel antegrade down the slow pathway while the fast pathway is still refractory; by the time the impulse reaches the lower region, the fast pathway recovers and conducts retrogradely, creating a loop that re-enters the atrium. This dual-pathway setup within the AV node is what sustains the rapid, regular tachycardia characteristic of AVNRT. The other options don’t fit because a single AV node lacks two separate routes for reentry, the AV annulus is more associated with accessory pathway–mediated tachycardias, and Purkinje fibers pertain to ventricular conduction rather than forming the AV nodal reentrant circuit.