The hallmark characteristic of a PAC (Premature Atrial Contraction) is?

A premature atrial contraction (PAC) is characterized by a premature, abnormally shaped P-wave. This occurs because the electrical impulse that causes the atria to contract originates from an ectopic focus in the atria rather than the sinoatrial (SA) node, which is the normal pacemaker of the heart. Consequently, the P-wave produced by the PAC appears different from the normal sinus P-waves and occurs earlier than expected in the cycle.

Understanding this characteristic is essential for correctly identifying PACs on an electrocardiogram (ECG). The other options described do not typically represent the defining features of PACs. For instance, a delayed T-wave or a dropped QRS complex are relevant to other types of arrhythmic events or conditions, while a prolonged PR interval signifies a delay in conduction through the atrioventricular (AV) node, which is not the primary issue in PACs.