This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
[{'content': 'P wave is absent \nSelect and explain. \nA. Atrial fibrillation, B. CCF, C. Atrial flutter, D. PSVT', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. (a) Atrial fibrillationRef: Ganong, 25th ed/p.524P wave is absent in atrial fibrillationInstead of p waves, only fibrillatory f waves are seenP wave is absent also in hyperkalemia', 'role': 'assistant'}]