Sequence of cardiac conduction events

Steps:

• SA node generates impulse, spreading to atria for depolarization.
• Atrial contraction follows as atria respond to impulse (event 3: D).
• Impulse reaches AV node, which delays it briefly (event 4; B misattributes to SA node).
• Post-delay, impulse conducts through bundle of His and Purkinje fibers to ventricles (event 5: A).

Why A is correct:

• Purkinje fibers transmit impulse rapidly to ventricular myocardium after AV nodal delay, per the cardiac conduction system's sequential pathway.

Why the others are wrong:

• B: Delay occurs at AV node, not SA node.
• C: SA node initiates impulse but does not directly contract atria.
• D: Atrial contraction is the third event, after SA depolarization.

Final answer: A