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
'a'wave in JVP is seen in- 
Select and explain. 
A. Will be large when atria contracts against resistance, B. Filling of ventricles, C. It is followed by "V" waves, D. in inspiration
A: Will be large when atria contracts against resistance
Answer: A: Will be large when atria contracts against resistance 
Explanation: The a wave is due to atrial systole.some blood regurgitates into great veins when the atria contract.In addition,venous flow stops, the resultant rise in venous pressure contributes to a wave. (Ganong&;s Reviw of medical physiology,23rd edition,pg no.512)