What landmark is targeted for transseptal puncture in left atrial EP procedures?

• A. The muscular septum between the atria
• B. The fossa ovalis of the interatrial septum
• C. The coronary sinus ostium
• D. The anterior mitral annulus

Answer: (B)

Access to the left atrium during EP procedures is gained by puncturing the interatrial septum at its thinnest, most reliable region. That region is the fossa ovalis, a shallow depression in the septum that marks the remnant of the fetal foramen ovale. Puncturing here, usually at the posterior-superior part of the fossa ovalis, provides a safer, easier entry into the left atrium and minimizes the risk to adjacent structures such as the aorta. The other structures are not appropriate landmarks for this puncture: the muscular part of the septum is thick and less safe to traverse; the coronary sinus ostium lies in the right atrium; and the anterior mitral annulus is part of the valve apparatus, not a septal entry point.