This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Medicine
A 62-year old man with carcinoma lung presented to emergency depament with respiratory distress. ECG showed electrical alternans. 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 alterans-a beat-to-beat alternation in one or more components of the ECG signal-is a common type of nonlinear cardiovascular response to a a variety of hemodynamic and electro physiologic peurbations.Total electrical alterans with sinus tachycardia is a relatively specific sign of pericardial effusion,usually with cardiac tamponade.</p Ref:Harrison's med-18th edition,page no:1838