What is unique about the left atrium's pectinate muscle?

• A. It spans both atria
• B. It has pectinate muscle only in its auricle
• C. It has more extensive pectinate muscles than the right atrium
• D. It lacks pectinate muscle entirely

Answer: (B)

Pectinate muscles are muscular ridges that line parts of the atrial walls and help increase surface area for contraction. In the left atrium, these ridges are confined to the left atrial appendage (auricle); the main body of the left atrium is smooth-walled. That makes the left atrium unique compared with the right atrium, where pectinate muscles extend into both the appendage and the atrial body. So the best statement is that pectinate muscle is present only in the left atrial appendage. The other ideas don’t fit: the pectinate muscles do not span both atria, the left atrium does not have more extensive pectinate muscles than the right, and it does not lack pectinate muscle entirely.