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
"Cannon" - a wave is seen in- 
Select and explain. 
A. Complete hea block, B. Left ventricular hyperophy, C. Mitral stenosis, D. atrial fibrillation
A: Complete hea block
Answer: A: Complete hea block 
Explanation: 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. The x descent defines the fall in right atrial pressure after inscription of the a wave. ref : ( Harrison&;s principle of internal medicine,20th edition,pg no.1823 )