Final answer:
The interval that begins at the QRS complex and ends at the apex of the T wave is called the ST interval. It represents the time between ventricular depolarization and repolarization. This interval is clinically relevant as it can indicate abnormal changes in the heart's electrical activity.
Step-by-step explanation:
The interval that begins at the QRS complex and ends at the apex of the T wave is not any of the options listed (PR interval, Corrected QT interval, Absolute refractory period, Relative refractory period). The interval in question is actually called the ST interval.
The ST interval represents the time between ventricular depolarization and repolarization. It is measured from the end of the QRS complex to the beginning of the T wave.
This interval is clinically relevant as it can indicate abnormal changes in the heart's electrical activity, such as myocardial ischemia or infarction.