Which layer of the pericardium is described as a tough protective membrane?

The correct choice is the fibrous pericardium, which serves as a tough protective membrane surrounding the heart. This outer layer is composed of dense connective tissue that provides structural support and helps anchor the heart within the mediastinum, preventing excessive movement. The fibrous pericardium also plays a critical role in protecting the heart from potential infection and trauma.

In contrast, the serous pericardium, which consists of two layers—the parietal layer that lines the fibrous pericardium and the visceral layer that covers the heart—serves primarily to reduce friction during heartbeats through the secretion of serous fluid. The pericardial fat layer aids in cushioning and insulating the heart but does not provide the same level of rigidity or protection as the fibrous pericardium. Lastly, the myocardium is the muscular layer of the heart itself, responsible for contracting and pumping blood, rather than serving as a protective membrane.