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
Pulsus paradoxus is characteristically seen in cardiac tamponade. Absence of pulsus paradoxus is seen when cardiac tamponade is associated with all except one 
Select and explain. 
A. ASD, B. VSD, C. MR, D. AR
C: MR
Answer: C: MR 
Explanation: Mechanism of pulsus paradoxus:
Inspiration → increased RV filling and decreased LV filling → left shift of IVS → LV compression and decreased systolic arterial pressure.
Pulsus paradoxus  is absent  when cardiac tamponade occurs in a condition which permits equal filling of both ventricles or more filling of LV.

ASD= equal filling of both ventricles in both phases of respiration.
VSD=free communication between ventricles preventing differential filling.
AR= filling of LV is maintained irrespective of respiration