All of the following are found in the right atrium except which structure?

• A. Coumadin ridge
• B. Crista terminalis
• C. Pectinate muscles
• D. SVC ostium

Answer: (A)

The main concept here is recognizing the standard internal landmarks of the right atrium. The right atrium contains a distinct muscular ridge called the Crista terminalis, which separates the smooth posterior sinus venarum from the rough anterior portion that has pectinate muscles. The pectinate muscles line the anterior right atrial wall and the right atrial appendage. The opening where the superior vena cava drains into the atrium is the SVC ostium, a normal feature of the right atrial interior.

The Coumadin ridge is a term used in some anatomical descriptions, but it is not a consistently described, fixed landmark of the right atrium. It is a variable or less-defined ridge and does not represent a standard, universally recognized structure like the Crista terminalis, pectinate muscles, or the SVC ostium. Because of its variability and lack of consistent recognition as a classic RA structure, it is considered the exception.