The triangle of Koch is an important surgical landmark for the AV node. Which three structures form its boundaries?

• A Crista terminalis, fossa ovalis, and Eustachian valve
• B Tendon of Todaro (superiorly), tricuspid valve annulus (anteroinferiorly), and orifice of the coronary sinus (posteriorly) ✓
• C Pulmonary valve, tricuspid valve, and right coronary artery
• D Moderator band, crista supraventricularis, and interventricular septum

Explanation

The triangle of Koch is a triangular area on the septal wall of the right atrium that overlies the AV node and the proximal part of the bundle of His. Its boundaries are: (1) anteriorly/superiorly — the tendon of Todaro (a fibromuscular ridge running from the Eustachian valve to the central fibrous body); (2) inferiorly — the hinge line (annulus) of the septal leaflet of the tricuspid valve; (3) posteriorly — the orifice of the coronary sinus. The apex of the triangle points toward the membranous interventricular septum where the bundle of His penetrates. This landmark is used in electrophysiology (AV nodal ablation) and septal defect repair.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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