**Core Concept**
The Koch's triangle, also known as the triangle of Koch, is a critical anatomical landmark in the right atrium that plays a key role in the conduction of the electrical impulse in the heart. It is bounded by three key structures: the tricuspid valve annulus, the tendon of Todaro, and the coronary sinus orifice. Understanding the boundaries of the Koch's triangle is essential for the diagnosis and management of various cardiac arrhythmias.

**Why the Correct Answer is Right**
The limbus fossa ovalis is not a boundary of the Koch's triangle. The limbus fossa ovalis is a remnant of the fetal foramen ovale, which closes after birth to form the interatrial septum. The Koch's triangle is specifically bounded by the tricuspid valve ring, the tendon of Todaro, and the coronary sinus orifice. The correct boundaries are essential for the precise placement of catheters and electrodes during electrophysiological studies.

**Why Each Wrong Option is Incorrect**
**Option A:** The tricuspid valve ring is indeed a boundary of the Koch's triangle, forming its anterior wall.
**Option B:** The coronary sinus orifice is a correct boundary of the Koch's triangle, forming its posterior wall.
**Option C:** The tendon of Todaro is also a correct boundary of the Koch's triangle, forming its inferior wall.

**Clinical Pearl / High-Yield Fact**
The Koch's triangle is a crucial landmark for the placement of the His bundle catheter during electrophysiological studies. The triangle is located on the anterior wall of the right atrium and is bounded by the tricuspid valve ring, the tendon of Todaro, and the coronary sinus orifice. This knowledge is essential for the precise identification of the His bundle and the diagnosis of various cardiac arrhythmias.

**✓ Correct Answer: D. Limbus fossa ovalis**