Which of the following best describes the appearance of premature atrial contractions on an EKG?

Premature atrial contractions (PACs) are characterized by early P waves that differ in morphology from the normal sinus P waves. When a PAC occurs, the ectopic atrial impulse leads to an atrial contraction that occurs sooner than expected within the cardiac cycle. This results in a P wave that is not preceded by a regular or typical sinus rhythm, thus appearing earlier than the regular sinus P waves.

The presence of early P waves is a key distinguishing feature of PACs, making this the best choice among the options provided. The normal QRS complexes typically remain unchanged, as PACs only affect the atrial activity and do not inherently alter the conduction through the ventricles. Irregular T waves and a prolonged QT interval are not characteristics directly associated with PACs and do not provide an accurate description of their appearance on an electrocardiogram.