Using a stethoscope, you listen to the heart of a patient in their early forties. You hear a clear 1st heart sound (lub), but the second heart sound is muffled and followed by a whooshing noise. What structure could be dysfunctional in this patient?

• A. Mitral valve.
• B. Right atrioventricular valve.
• C. Aortic valve.
• D. Both A & B could be the cause.