Final answer:
Electrical Alternans on an EKG indicate a swinging motion of the heart, which is most commonly associated with a large pericardial effusion, detectable via an echocardiogram.
Step-by-step explanation:
When an EKG shows Electrical Alternans, the most likely finding on the echocardiogram would be a large pericardial effusion. Electrical Alternans is characterized by alternating changes in the amplitude of the QRS complexes, which may suggest a swinging motion of the heart within a large pericardial effusion. An echocardiogram, being an ultrasound test of the heart, helps in assessing the effectiveness of the heart's pumping action and can visualize the accumulation of fluid in the pericardial cavity.