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': "'a' wave in JVP Indicates \nSelect and explain. \nA. Right atrial contraction, B. Closure of tricuspid valve, C. Onset of ventricular systole, D. Maximal atrial filling", 'role': 'user'}, {'content': 'Answer: A \nExplanation: (a) Right atrial contractionRef: Harrison, 18th ed.,pg. 1823, fig. 227-1Graph of height of JVP with timeThe upward deflections are the"a" atrial contraction"c" ventricular contraction and resulting bulging of tricuspid into the right atrium during isovolumetric systole)"v" = venous fillingThe downward deflections of the wave are the"x" the atrium relaxes, and the tricuspid valve moves downwardthe "y" descent filling of ventricle after tricuspid opening.', 'role': 'assistant'}]