This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, Subject name: Medicine
[{'content': "'a' wave is absent in \nSelect and explain. \nA. Atrial fibrillation, B. Heart block, C. Tricuspid regurgitation, D. Pericardial effusion", 'role': 'user'}, {'content': "Answer: A \nExplanation: (A) # 'a' wave of JVP: Due to right atrial contraction Synchronous with carotid artery pulse> Absent 'a' wave: Atrial fibrillation> Diminished 'a' wave: Tachycardia; Prolonged PR interval> 'a' Wave Fused with 'c' Wave: Supra ventricular tachycardia> Large 'a' Waves: Pulmonary hypertension Pulmonary stenosis; Tricuspid stenosis Tricuspid atresia; AV dissociation> Cannon Waves (Giant 'a' Waves): Complete heart block when right atrium occasionally contracts against closed tricuspid valve Ventricular tachycardia; Ectopic beats", 'role': 'assistant'}]