Which feature best distinguishes a thrombus from an embolus in clinical and pathological terms?

Explanation

## Distinguishing Thrombus from Embolus ### Key Pathological Difference **Key Point:** The fundamental distinction between a thrombus and an embolus is **where they form versus where they lodge**: - **Thrombus**: Forms *in situ* — the site of formation and the site of lodgement are **the same** (e.g., a coronary artery thrombus forms at the site of an atherosclerotic plaque rupture and occludes that same vessel) - **Embolus**: Forms at one location (the source) and travels through the circulation to lodge at a **different, distant site** — the site of formation and the point of lodgement are **different** ### Why Option A Is Correct **High-Yield:** Option A states "Site of formation and point of lodgement are the same" — this is the defining feature of a **thrombus**, and by contrast, the defining feature of an **embolus** is that these two sites are *different*. This single criterion best distinguishes the two entities in both clinical and pathological terms (Robbins & Cotran Pathologic Basis of Disease, 10e, Ch. 4). ### Comparative Table | Feature | Thrombus | Embolus | |---------|----------|---------| | **Site of formation = Site of lodgement?** | ✅ Yes (forms in situ) | ❌ No (travels from distant source) | | **Mechanism** | Local endothelial injury, stasis, or hypercoagulability (Virchow's triad) | Detached material travels through circulation | | **Composition** | Fibrin, platelets, RBCs — lines of Zahn | Variable: thromboembolus, fat, air, amniotic fluid, tumour | | **Vessel type** | Arteries or veins | Lodges where vessel narrows | ### Why Option C Is Incorrect **Clinical Pearl:** Option C ("Always originates from a distant source and travels through the circulation") describes an embolus, not the distinguishing feature of a thrombus vs. embolus comparison. Moreover, the word **"always"** makes it factually imprecise — while most emboli do originate from distant sources, the absolute qualifier is problematic. More importantly, Option C describes only the embolus, whereas Option A correctly identifies the key distinguishing criterion that separates a thrombus (forms in situ) from an embolus (lodges away from origin). ### Why Other Options Are Wrong - **Option B** (composed entirely of fibrin without cellular elements): Incorrect — thrombi contain fibrin, platelets, and RBCs (lines of Zahn); they are not purely fibrin. - **Option D** (forms only in veins and never in arteries): Incorrect — thrombi can form in both arteries (e.g., coronary artery thrombosis) and veins (e.g., DVT). ### Mnemonic **Mnemonic:** **THROMBUS = There, Here, Right On My Blood Vessel's Usual Site** — emphasizes that a thrombus forms exactly where it causes obstruction, unlike an embolus which travels from elsewhere. [cite:Robbins 10e Ch 4]