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
Cannon ‘a’ wave in venous waveform is suggestive of 
Select and explain. 
A. AV dissociation, B. Atrial fibrillation, C. Tricuspid incompetency, D. All of the above
A: AV dissociation
Answer: A: AV dissociation 
Explanation: 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, preceding 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