Which set of structures defines the Triangle of Koch?

• A. Septal leaflet of TV valve; CS ostium; Tendon of Todaro
• B. Septal leaflet of TV valve; Fossa ovalis; Tendon of Todaro
• C. CS ostium; Fossa ovalis; Tendon of Todaro
• D. Septal leaflet of TV valve; CS ostium; Tendon of Todaro; All of the above

Answer: (D)

The Triangle of Koch defines where the AV node sits in the right atrium, so its boundaries are the three landmarks you can reliably use to locate that node. Those boundaries are the tendon of Todaro, the ostium of the coronary sinus, and the septal leaflet of the tricuspid valve. The AV node sits at the apex of this triangle, near the interatrial septum.

The fossa ovalis lies on the interatrial septum nearby, but it is not a boundary of Koch’s triangle. So a description that includes the fossa ovalis as part of the triangle isn’t the standard anatomical definition. Therefore, the commonly accepted description is the trio of tendon of Todaro, coronary sinus ostium, and septal leaflet of the tricuspid valve.