Reentry occurs when there is which condition in a conduction pathway?

• A. Partial depolarization or unidirectional block
• B. Complete depolarization
• C. Hyperpolarization
• D. No conduction delay

Answer: (A)

Reentry requires a circulating impulse around a circuit, which is made possible when part of the pathway has a block in one direction while remaining excitable in the other. This unidirectional block, coupled with slowed conduction in the surrounding tissue, allows the wavefront to travel around the circuit and re-enter tissue after it has recovered from its refractory period, sustaining the arrhythmia. Partial depolarization in a region can predispose to this situation by reducing available sodium channels and slowing conduction, creating a block in one direction. Once the tissue ahead recovers, the previously circulating impulse can re-enter and continue propagating.

If tissue were completely depolarized, it would be unable to conduct; hyperpolarization tends to enhance excitability rather than create the necessary blocking and timing differences; and a pathway with no conduction delay wouldn’t provide the needed timing gap for the cycle to complete.