During cardiac embryology, the sinus venosus is initially a paired structure. Which sinus venosus-derived structure gives rise to the smooth part of the right atrium (sinus venarum)?

• A Right horn of sinus venosus ✓
• B Left horn of sinus venosus
• C Primordial atrium (trabeculated part)
• D Bulbus cordis

Explanation

The right horn of the sinus venosus enlarges and is incorporated into the right atrium to form the smooth-walled sinus venarum (posterior wall between the venae cavae). The left horn of the sinus venosus is incorporated into the right atrium as the coronary sinus. The trabeculated right atrium (right auricle) represents the original primordial atrium. The crista terminalis (a muscular ridge internally, corresponding to the sulcus terminalis externally) marks the junction between the smooth sinus venarum and the trabeculated primordial atrium.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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