A 58-year-old man from Delhi presents to the emergency department with acute chest pain radiating to the left arm, onset 2 hours ago. He has a 15-year history of type 2 diabetes mellitus (HbA1c 9.2%), hypertension (BP 165/95 mmHg), and smokes 20 cigarettes daily. His father died of myocardial infarction at age 62. Serum lipid profile shows total cholesterol 285 mg/dL, LDL-C 210 mg/dL, HDL-C 28 mg/dL, triglycerides 380 mg/dL. ECG shows ST elevation in leads II, III, and aVF. Coronary angiography reveals a thrombotic occlusion of the right coronary artery with underlying atherosclerotic plaque. Histopathology of the aspirated thrombus and plaque material shows lipid-rich core with a thin fibrous cap infiltrated by macrophages and T lymphocytes. Which of the following pathological features best explains the acute thrombotic event in this patient?

Explanation

## Pathological Mechanism of Acute Atherothrombotic Events ### The Vulnerable Plaque Concept **Key Point:** Acute coronary syndromes typically result from rupture of a **thin-capped fibroatheroma (TCFA)** with exposure of the highly thrombogenic lipid core, not from progressive stenosis alone. ### Histopathological Features of Rupture-Prone Plaques | Feature | Vulnerable Plaque | Stable Plaque | |---------|-------------------|---------------| | **Lipid core** | Large, lipid-rich | Small, minimal | | **Fibrous cap** | Thin (<65 μm) | Thick (>200 μm) | | **Inflammatory infiltrate** | Dense macrophages, T cells | Sparse | | **Smooth muscle cells** | Few, inactive | Abundant, active | | **Tissue factor content** | High (in lipid core) | Low | | **Thrombotic risk** | Very high | Low | ### Why Plaque Rupture Causes Thrombosis 1. **Exposure of tissue factor (TF)**: The lipid core is rich in TF, a potent initiator of the extrinsic coagulation cascade. 2. **Platelet adhesion**: Von Willebrand factor and collagen in the exposed subendothelial matrix trigger platelet aggregation. 3. **Thrombin generation**: TF + VIIa complex activates the coagulation cascade, generating thrombin and fibrin. 4. **Occlusive thrombus**: Rapid thrombus formation can completely occlude the vessel, causing acute MI. **High-Yield:** The **thin fibrous cap infiltrated by macrophages and T lymphocytes** described in this case is the hallmark of a vulnerable plaque. Macrophages produce matrix metalloproteinases (MMPs) that degrade collagen and weaken the cap, predisposing to rupture. ### Clinical Risk Factors in This Patient **Key Point:** Multiple factors accelerate atherosclerosis and plaque vulnerability: - **Diabetes**: Increases oxidative stress, inflammation, and endothelial dysfunction. - **Smoking**: Promotes oxidative stress, increases thrombogenicity, and impairs endothelial repair. - **Dyslipidemia**: High LDL and low HDL drive lipid accumulation and inflammation. - **Hypertension**: Causes mechanical shear stress and endothelial injury. - **Family history**: Genetic predisposition to accelerated atherosclerosis. ### Distinction from Other Mechanisms **Clinical Pearl:** While endothelial erosion (option C) can cause acute thrombosis in some cases (especially in young women and diabetics), it accounts for only ~30% of acute coronary events. Plaque rupture is responsible for ~70% of cases and is the mechanism in this patient with a large lipid core and thin cap. [cite:Robbins 10e Ch 11]