<prompt> In the JVP, a waves are absent in -Atrial fibrillationMitral stenosisTricuspid atresiamyocardial infarction  <response>: The a wave reflects right atrial presystolic contraction and occurs just after the electrocardiographic P wave, preceding the first hea sound S1.A prominent a wave is seen in patients with reduced right ventricular compliance; a cannon a wave occurs with atrioventricular AV dissociation and right atrial contraction against a closed tricuspid valve. In a patient with a wide complex tachycardia, the appreciation of cannon a waves in the jugular venous waveform identifies the rhythm as ventricular in origin. The a wave is not present with atrial fibrillation. Ref Harrisons internal medicine 20e p1668 