Endocardial cushion defects are associated with which condition?

Endocardial cushion defects are primarily associated with Down syndrome, also known as trisomy 21. This condition is characterized by the presence of an extra chromosome 21, which leads to various congenital heart defects, including atrioventricular septal defects (endocardial cushion defects).

These defects result from improper development of the heart's septal structures and can lead to significant clinical implications, such as heart failure and pulmonary hypertension if not addressed. While other syndromes like Marfan syndrome and Turner syndrome can lead to different types of cardiac anomalies, they are not specifically linked to endocardial cushion defects.

In the context of the options, choice D is more precise as it directly references the condition associated with endocardial cushion defects, indicating its stronger connection to Down syndrome or trisomy 21.