When does the U wave occur during the cardiac cycle?

The U wave is generally recognized as a deflection occurring in the electrocardiogram after the T wave, primarily associated with the repolarization of the Purkinje fibers and possibly the myocardium of the ventricles. This wave is not commonly observed in every ECG; however, when it is present, it signifies the final stage of ventricular repolarization.

The Purkinje fibers are specialized conductive fibers that distribute the electrical impulse throughout the ventricles, leading to an efficient contraction. Following the T wave, which reflects the repolarization of the ventricles, the U wave indicates that this process has continued, particularly concerning the Purkinje system's recovery.

The other options are aligned with various aspects of the cardiac cycle but do not correctly pertain to the specific activity represented by the U wave. For instance, atrial dilation occurs prior to the contraction and does not relate to the repolarization process. Similarly, ventricular contraction corresponds to the QRS complex, not the U wave, while atrial relaxation is linked to the end of the P wave and does not involve the repolarization of the ventricles or Purkinje fibers.