Final answer:
The fastest conduction of an action potential through the heart's conduction system occurs in the Purkinje fibers, allowing rapid and coordinated contraction of the ventricles.
Step-by-step explanation:
The propagation of the action potential through the heart varies in speed across different components of the cardiac conduction system. The correct answer to the question is (D) Purkinje fibers, which have the fastest conduction velocity, allowing the electrical impulse to reach all of the ventricular muscle cells rapidly, approximately in 75 ms. In contrast, the atrioventricular node (AV node) has a slower conduction rate because it provides a critical delay (approximately 100 ms) to ensure the atria have enough time to pump blood into the ventricles before ventricular contraction begins. This orchestrated sequence ensures efficient blood flow through the heart and into the systemic and pulmonary circuits.