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
[{'content': 'A 62-year-old male with underlying COPD develops a viral upper respiratory infection and begins taking an over-the-counter decongestant. Shortly thereafter he experiences palpitations and presents to the emergency room, where the following rhythm strip is obtained, demonstrating \nSelect and explain. \nA. Normal sinus rhythm, B. Junctional rhythm, C. Atrial flutter with 4:1 atrioventricular block, D. Paroxysmal atrial tachycardia with 2:1 atrioventricular block', 'role': 'user'}, {'content': 'Answer: C \nExplanation: (c) Source: (Braunwald, 15/e, p 1297.) The rhythm strip in the question reveals atrial flutter with 4:1 atrioventricular (AV) block. Atrial flutter is characterized by an atrial rate of 250 to 350/min; the electrocardiogram typically reveals a sawtooth baseline configuration due to the flutter waves. In the strip, every fourth atrial depolarization is conducted through the AV node, resulting in a ventricular rate of 75/min (although 2:1 conduction is more commonly seen).', 'role': 'assistant'}]