Cardiac Conduction Sequence

Steps:

• Sinoatrial node fires, depolarizing atria and causing contraction (event 1).
• Electrical impulse reaches atrioventricular node for delay, allowing atrial emptying (event 4).
• Impulse travels via bundle of His to Purkinje fibers, rapidly spreading through ventricles (event 3).
• Ventricular depolarization triggers left and right ventricle contraction (event 2).

Why B is correct:

• B follows the conduction pathway: atrial depolarization (1) → AV node delay (4) → Purkinje conduction (3) → ventricular contraction (2), per cardiac electrophysiology.

Why the others are wrong:

• A: Places Purkinje conduction (3) before AV node (4), inverting the bundle of His sequence.
• C: Starts with ventricular contraction (2), but atria contract first after SA node.
• D: Positions AV node (4) before atrial contraction (1), but atria depolarize immediately after SA node.

Final answer: B