Final answer:
The most common abnormality that needs to be constantly monitored in exercise testing as a diagnostic tool is dysfunction of the heart.
Step-by-step explanation:
In exercise testing used as a diagnostic tool, the most common abnormality that needs to be constantly monitored is dysfunction of the heart. Exercise testing, such as stress tests, can help identify abnormal heart rhythms (arrhythmias), inadequate blood flow to the heart (ischemia), and other heart-related abnormalities.