Endocardial fibroelastosis (EFE) is a rare cardiac disorder characterized by diffuse thickening of the ventricular endocardium with dense collagen and elastin deposition. The fibrotic layer predominantly affects the left ventricle, creating a smooth, glistening, porcelain-white endocardial surface that significantly impairs cardiac function through two mechanisms: (1) restricted diastolic filling (the thick, non-compliant endocardium prevents normal ventricular relaxation and filling, creating restrictive physiology) and (2) impaired systolic contraction (the fibrotic layer acts as a rigid shell that limits myocardial shortening). EFE primarily affects infants and young children, with two distinct forms: primary (dilated) EFE presents with a dilated left ventricle and reduced ejection fraction (most common form, ~80% of cases), while secondary (contracted) EFE presents with a small, restrictive left ventricle and is usually associated with left-sided obstructive lesions (aortic stenosis, coarctation, hypoplastic left heart syndrome). The etiology involves viral myocarditis (mumps and coxsackievirus B are strongly implicated), genetic factors (autosomal recessive inheritance patterns identified in familial cases), and fetal endocardial ischemia from impaired coronary perfusion. The fibrotic endocardium also distorts the mitral and aortic valve apparatus, frequently causing mitral regurgitation...