This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Medicine
A 62 years old man with carcinoma of lung presented to emergency depament with respiratory distress. His EKG showed electrical alternans. The most likely diagnosis is 
Select and explain. 
A. Pneumothorax., B. Pleural effusion, C. Cardiac tamponade, D. Constrictive pericarditis.
C: Cardiac tamponade
Answer: C: Cardiac tamponade 
Explanation: Electrical alternans is an electrocardiographic 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 effusionMassive pericardial effusion produces a triad of:Low QRS voltageTachycardiaElectrical alternansRef: Harrison's 18/e p1972