Which electrical activity occurs just before the ventricles contract?

The electrical activity that occurs just before the ventricles contract is reflected in the QRS complex. This is a crucial part of the cardiac cycle. The QRS complex represents the depolarization of the ventricles, which triggers their contraction.

To clarify the options: atrial depolarization actually occurs earlier in the cardiac cycle and is represented by the P wave of the EKG. Ventricular repolarization is indicated by the T wave and takes place after the ventricles contract. The PR interval is the time from the beginning of atrial depolarization to the beginning of ventricular depolarization, which includes the time the electrical impulse takes to travel through the AV node and into the ventricles.

Thus, the correct understanding of the timeline of electrical activity in the heart indicates that the QRS complex marks the moment just prior to ventricular contraction.