What structure can hinder access to the coronary sinus ostium?

• A. Thebesian valve
• B. Eustachian valve
• C. Pituitary valve
• D. Mitral valve

Answer: (A)

The structure is the Thebesian valve, a small flap that can sit right at the opening of the coronary sinus into the right atrium. When this valve is present, it can partially or fully cover the coronary sinus ostium, making cannulation difficult during EP procedures. Recognizing a Thebesian valve helps explain why a catheter may resist entering the coronary sinus and guides gentle catheter manipulation or a slight adjustment in approach to locate a path into the sinus beyond the valve. The Eustachian valve sits at the IVC ostium and is not the structure guarding the coronary sinus ostium; the mitral valve is separate and not involved in accessing the coronary sinus.