This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': '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 : \nSelect and explain. \nA. Pneumothorax, B. Pleural effusion, C. Cardiac tamponade, D. Constrictive pericarditis.', 'role': 'user'}, {'content': "Answer: C \nExplanation: Answer is C (Cardiac tamponade): Electrical alternans refers to a beat-to-beat alteration in one or more components of the ECG signal. Total electrical alternans (P-QRS-T) with sinus tachycardia is a relatively specific sign for pericardial effusion, often with cardiac tamponade. - Harrison /5'4/1270 Feature Cardiac Tamponade Constrictive Pericarditis Restrictive Cardiomyopathy RVMI Electrical Alternans May he present Absent Absent Absent Note: Pleural effusion and pneumothorax both are potential complications of carcinoma lung that present with respiratory distress but electrical alternans is not associated.", 'role': 'assistant'}]