Which of the following is NOT one of the three layers of the heart wall?

The pericardium is indeed not one of the three layers that make up the heart wall. The three primary layers of the heart wall are the endocardium, myocardium, and epicardium.

The endocardium is the innermost layer, lining the heart's chambers and valves, providing a smooth surface for blood flow. The myocardium is the middle layer and is primarily composed of cardiac muscle, responsible for the heart's contracting action, which pumps blood throughout the body. The epicardium is the outer layer, which serves as a protective layer and also contains blood vessels, nerves, and lymphatics.

The pericardium, on the other hand, is a double-walled sac that surrounds and protects the heart but does not form part of the heart wall itself. Being an important structure, it serves to anchor the heart and prevent friction during heartbeats, but it is distinctly separate from the heart’s three layers. Thus, identifying the pericardium as not being part of the heart wall is correct.