Answer: Several arteries in the sections showed this feature. What is it? A thrombus or thromboembolus

What is the irregularly-shaped, pink structure marked by the asterisk? It is the part of a thrombus known as a line of Zahn. These are platelet-leukocyte strands that form in circulating blood as it coagulates. They can be seen in thrombi with the naked eye. Stagnant blood that clots becomes layered into a mass of RBCs ("currant jelly") and plasma and WBCs ("chicken fat"). The lines of Zahn in the thrombus constitute its architecture, i.e., the specific feature that distinguishes it from stagnant clotted blood. This thrombus is recent because, although it is adherent to the vessel wall, it shows no evidence of organization or recanalization yet.

In situ thrombi may develop in small, muscular arteries in acute inflammatory conditions in the lung, but such thrombi cannot be distinguished from thromboemboli from another source.

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