Which combination is required for a reentrant circuit to exist?

• A. Two pathways with different conduction velocities and refractory periods
• B. Two pathways connected proximally and distally to form a circuit
• C. A unidirectional block in one of the pathways
• D. All of the above

Answer: (D)

Reentry requires an impulse to keep circulating around a loop and re-excite tissue after it has recovered from refractoriness. This needs three pieces working together: two pathways that the impulse can travel through, with different conduction velocities and refractory properties so the timing around the loop isn’t uniform; these pathways must be connected to form a closed circuit so the impulse can return to its starting point; and a unidirectional block in one pathway to allow the wavefront to go around the loop in one direction but not in the opposite direction, letting the tissue ahead recover and be ready to be re-excited. When all three conditions are present, a self-sustaining reentrant circuit can exist.