Endocardium is the smooth inner lining.

• A. Outer surface of the heart
• B. Thick middle layer made of muscle
• C. Double membrane sac outside the heart
• D. Smooth inner lining

Answer: (D)

Endocardium is the smooth inner lining of the heart's chambers and valves. It’s a thin layer of endothelial cells that provides a slick surface to minimize friction as blood moves through the heart and helps protect the delicate structures inside. It remains continuous with the endothelium lining of all blood vessels.

The outer surface of the heart is the epicardium (often referred to as the visceral layer of the pericardium), the thick muscular middle layer is the myocardium, and the double-membrane sac surrounding the heart is the pericardium. Because endocardium specifically describes the innermost lining, “smooth inner lining” is the correct description.