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': 'Cannon ‘a’ wave in venous waveform is suggestive of \nSelect and explain. \nA. AV dissociation, B. Atrial fibrillation, C. Tricuspid incompetency, D. All of the above', 'role': 'user'}, {'content': "Answer: A \nExplanation: The venous waveform is divided into several distinct peaks. The 'a' wave reflects right atrial presystolic contraction and occurs just after the electrocardiographic P wave,\xa0preceding the first heart sound (S). A prominent 'a' wave is seen in patients with reduced right ventricular compliance; cannon 'a' wave occurs with atrioventricular (AV) dissociation", 'role': 'assistant'}]