Distinguish a thrombus from an embolus.

• A. An embolus is a clot that travels and lodges elsewhere.
• B. A thrombus is a clot that forms in a vessel; an embolus travels elsewhere.
• C. They are infections.
• D. They are the same thing.

Answer: (B)

A thrombus forms in a vessel at the spot where it starts, growing there and potentially blocking flow locally. An embolus, on the other hand, is material that travels through the bloodstream from somewhere else and can lodge in a distant vessel, causing an obstruction there. This distinction is exactly what the statement captures: a thrombus is a clot that forms in a vessel, while an embolus travels elsewhere. For example, a clot forming in a leg vein is a thrombus; if a piece breaks off and travels to the lungs, it becomes an embolus causing a pulmonary embolism. They’re related concepts but describe different processes, and infections aren’t what these terms refer to.