Questions: Which part of the heart's beat is represented by the QRS complex on a patient's electrocardiogram (ECG)? Depolarization of the atria Repolarization of the ventricles Depolarization from the atrioventricular (AV) node through the ventricles The length of time for the impulse to travel from the atria to the ventricles

The answer is the third one: Depolarization from the atrioventricular (AV) node through the ventricles.

Explanation for each option:

1. Depolarization of the atria: This is incorrect. The depolarization of the atria is represented by the P wave on an ECG, not the QRS complex.

2. Repolarization of the ventricles: This is incorrect. The repolarization of the ventricles is represented by the T wave on an ECG.

3. Depolarization from the atrioventricular (AV) node through the ventricles: This is correct. The QRS complex on an ECG represents the depolarization of the ventricles, which occurs as the electrical impulse travels from the AV node through the ventricles.

4. The length of time for the impulse to travel from the atria to the ventricles: This is incorrect. The time it takes for the impulse to travel from the atria to the ventricles is represented by the PR interval, not the QRS complex.

In summary, the QRS complex on an ECG represents the depolarization of the ventricles as the electrical impulse travels from the AV node through the ventricles.