Pre-excitation produces a characteristically abnormal amount of which wave and short PR interval?

Pre-excitation refers to a condition where there is an accessory pathway for electrical impulses to bypass the normal conduction system of the heart, leading to early activation of the ventricles. This condition is commonly associated with syndromes like Wolff-Parkinson-White (WPW) syndrome.

The QRS complex represents the electrical depolarization of the ventricles in an EKG tracing. In cases of pre-excitation, the accessory pathway allows for a faster conduction of impulses from the atria to the ventricles, resulting in a wide and often distorted QRS complex. This is due to the ventricles being activated earlier than they would be through the normal conduction pathway, leading to a shortened PR interval. The PR interval is the time from the beginning of atrial depolarization to the beginning of ventricular depolarization; when pre-excitation occurs, this interval is shorter because the signal reaches the ventricles more quickly.

Thus, its characteristic effect is the abnormality in the QRS complex coupled with a short PR interval, making it the correct answer.