To sustain a reentry circuit, which of the following is required?

• A. One pathway has longer refractory period
• B. One pathway conducts slower than the other
• C. Two Parallel Pathways Connect Proximally And Distally
• D. All of the above

Answer: (D)

Sustained reentry relies on a looping circuit that surrounds an obstacle, with tissue in the loop having differences in both conduction time and excitability. If one limb of the loop has a longer refractory period, the impulse circulating around the circuit can block in one direction yet still propagate in the other, creating a unidirectional block that lets the wavefront continue around the loop. Slower conduction in one pathway increases the time the impulse spends in the circuit, helping ensure that tissue ahead has recovered from refractoriness, so the wavefront can re-excite it and keep the loop going. Having two parallel pathways that connect at proximal and distal points establishes the actual circuit around the obstacle, providing a path for the impulse to travel continuously and return to its starting point. When these features coexist—a loop around an obstacle, differing refractoriness, and differential conduction—the reentry circuit can be sustained.