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': 'Canon A wave is seen in: \nSelect and explain. \nA. Atrial fibrillation, B. Complete heart block, C. Ventricular fibrillation, D. Mobitz 1 heart block', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. (b) Complete heart blockRef.-.Harrison 18th edition, chapter 227:* The a wave reflects right atrial presystolic contraction and occurs just after the electrocardiographic P wave, preceding the first heart sound (Sj).* The a wave is not present with atrial fibrillation.* Canon a wave occurs with# Atrioventricular (AV) dissociation/ complete heart block due to right atrial contraction against a closed tricuspid valve.# Wide complex tachycardia like ventricular tachycardia* A prominent a wave is seen in patients with reduced right ventricular compliance like Right ventricular failure.', 'role': 'assistant'}]