This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Medicine
Electrical alternans is seen in - 
Select and explain. 
A. Cardiac tamponade, B. Restrictive cardiomyopathy, C. Constrictive pericarditis, D. Right Ventricular MI (RVMT
A: Cardiac tamponade
Answer: A: Cardiac tamponade 
Explanation: Electrical alternans is an electrocardiac phenomenon of alternation of QRS complex amplitude or axis between beats and a possible wandering base-line. It is seen in cardiac tamponade and severe pericardial effusion and is thought to be related to changes in the ventricular electrical axis due to fluid in the pericardium , as the hea essentially wobbles in the fluid filled pericardial sac . Ref Harrison 20,th edition pg 1543