Which of the following is a hallmark of premature ventricular contraction (PVC)?

Premature ventricular contractions (PVCs) are characterized by a specific morphology of the QRS complex due to their origin in the ventricles rather than the atria. The hallmark feature of a PVC is that the QRS complex appears wider than normal. This is because PVCs occur when the heart's conduction system bypasses the usual pacemaking pathways and activates the ventricles in a manner that is different from normal conduction. As a result, the electrical signal takes longer to traverse the ventricles, leading to a wider QRS complex, often measuring 0.12 seconds or greater.

The other characteristics listed are not specific to PVCs. Normally, a P wave precedes a QRS complex in standard sinus beats, but in the case of a PVC, there is typically no preceding P wave, as the ventricles are activated earlier than the next atrial impulse would occur. The T wave's direction can vary; while it might sometimes move in the same direction as the QRS complex, this is not a defining feature of PVCs. Therefore, the defining characteristic of PVCs remains the wide QRS complex, distinguishing them from other types of premature contractions or arrhythmias.