The Eustachian ridge is a remnant of which valve and where is it located?

• A. A remnant of the Thebesian valve on the floor of the right atrium
• B. A remnant of the Eustachian valve located posterior to the coronary sinus on the floor of the right atrium
• C. A muscular ridge near the fossa ovalis
• D. A fibrous ring around the tricuspid valve

Answer: (B)

The Eustachian ridge represents the adult remnant of the Eustachian valve. In fetal life, this valve directs blood from the inferior vena cava toward the foramen ovale. After birth it largely regresses, leaving a ridge along the floor of the right atrium. This ridge sits just behind the coronary sinus opening, i.e., posterior to the coronary sinus on the floor of the right atrium. That’s why the correct description is a remnant of the Eustachian valve located posterior to the coronary sinus on the floor of the right atrium. The Thebesian valve would be at the coronary sinus ostium itself, and the crista terminalis is a different muscular ridge, not the Eustachian remnant.