USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

The characteristic of failure of endocardial cushion fusion is the presence of an ostium primum atrial septal defect (ASD). This condition arises from improper fusion of the endocardial cushions during embryonic heart development, which is crucial for forming the septa and valves of the heart. When this fusion does not occur properly, it leads to defects such as an ostium primum ASD, which is a type of atrial septal defect located at the lower part of the atrial septum and is often associated with atrioventricular septal defects and single atrioventricular valves. Other options such as a mid-systolic rumble pertain to various types of heart murmurs and do not directly relate to the structural changes caused by endocardial cushion defects. Similarly, a single AV valve can occur in more complex forms of congenital heart disease involving complete atrioventricular septal defects, but the primary characteristic associated with failure of endocardial cushion fusion is indeed the ostium primum ASD, as it directly results from the lack of proper fusion of these structures. Increased pulmonary vascular resistance is typically a consequence of longstanding left-to-right shunts or other forms of heart failure, not a direct characteristic of endocard