Embryology of Atrial septal Defects

The correct answer is C.

Explanation:

The septum primum (first interatrial septum) develops by growing from the cranial end of the embryonic atrium toward the endocardial cushions. The gap that exists between the two atria during this period is the ostium primum. As the septum primum continues its growth, the ostium primum gets smaller. When the septum primum completes its growth and completely fuses with the endocardial cushions, the ostium primum is closed. Failure of the septum primum to fuse completely with the endocardial cushions leaves a persistent ostium primum, known as a primum type atrial septal defect.

The ostium primum does not form within the septum primum (choice A). The ostium primum is the communication between the two atria that exists during the formation of the septum primum. That is, the ostium primum is the space within the developing atrium not yet occupied by the septum primum.

The ostium secundum normally forms within the septum primum (choice B) before the ostium primum closes by fusion of the septum primum with the endocardial cushions. Failure of the ostium secundum to form would result in embryonic death, because there would be no pathway for blood to pass from the right atrium to the left atrium when the ostium primum closes, thus depriving the embryo of oxygenated blood.

Most of the septum primum normally disappears. The part that remains forms the valve of the foramen ovale. This part of the septum primum normally does not fuse with the septum secundum (choice D) during prenatal life. After birth, the valve of the foramen ovale is pushed against the septum secundum as a result of the increased pressure in the left atrium. This achieves functional closure of the foramen ovale. Fusion does not normally occur at this time, but usually occurs later in life in most people. In some people, however, complete fusion never occurs (probe patency).

The septum secundum normally does not fuse with the endocardial cushions (choice E).

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