Valve closure at onset of ventricular systole

Steps:

• Identify X as the point where left ventricular pressure begins rising above atrial pressure.
• Recall that atrioventricular (AV) valves close when ventricular pressure exceeds atrial pressure to prevent backflow.
• Confirm this marks the start of isovolumetric contraction in the cardiac cycle.
• Rule out semilunar valve actions, which depend on aortic pressure comparisons.

Why A is correct:

• AV valves close when ventricular pressure surpasses atrial pressure, initiating systole per cardiac cycle physiology.

Why the others are wrong:

• B: AV valves open during ventricular diastole for filling, before pressure rise at X.
• C: Semilunar valves close at end-diastole when ventricular pressure falls below aortic.
• D: Semilunar valves open later in systole when ventricular pressure exceeds aortic pressure.

Final answer: A