Which is the correct sequence of cardiac conduction starting from the sinoatrial node?

• A. AV node → SA node → bundle of His → Purkinje fibers → right/left bundles
• B. SA node → Purkinje fibers → AV node → bundle of His → right/left bundles
• C. AV node → Purkinje fibers → bundle of His → SA node → right/left bundles
• D. SA node → AV node → AV bundle → right and left bundle branches → Purkinje fibers

Answer: (D)

The main idea is the heart’s conduction pathway from the natural pacemaker to the ventricles: the impulse starts in the sinoatrial node, travels to the atrioventricular node where there’s a brief delay to allow ventricular filling, then moves down the AV bundle (Bundle of His) into the right and left bundle branches, and finally spreads through the Purkinje fibers to the ventricular myocardium for coordinated contraction. This sequence—SA node → AV node → AV bundle → right and left bundle branches → Purkinje fibers—fits the anatomical order and the functional roles: the AV node slows the signal, the Bundle of His conducts it to the ventricles, the branches deliver it into the septum, and the Purkinje network rapidly depolarizes the ventricles. Other sequences misplace components (for example, sending Purkinje fibers before the AV node or placing the SA node after downstream structures), which would disrupt the proper timing and flow of excitation.