Which histopathological feature best distinguishes a stable atherosclerotic plaque from a vulnerable (rupture-prone) plaque?

Explanation

## Distinguishing Stable from Vulnerable Plaques ### Vulnerable Plaque Characteristics **Key Point:** A vulnerable (rupture-prone) plaque is characterized by a **large lipid-rich necrotic core covered by a thin, weak fibrous cap**. This architecture is inherently unstable and prone to rupture, leading to acute thrombotic events. ### Structural Comparison | Feature | Vulnerable Plaque | Stable Plaque | |---------|-------------------|---------------| | **Lipid core size** | Large, extensive | Small or minimal | | **Fibrous cap** | Thin (<65 μm), weak | Thick (>65 μm), robust | | **Smooth muscle cells** | Sparse, apoptotic | Abundant, intact | | **Macrophage infiltration** | Heavy (>25% of cap) | Minimal | | **Collagen content** | Low, disorganized | High, organized | | **Clinical outcome** | Rupture → acute MI/stroke | Gradual stenosis | ### Pathophysiology of Vulnerability 1. **Thin fibrous cap** — composed mainly of collagen types I and III; degraded by matrix metalloproteinases (MMPs) released by activated macrophages. 2. **Large lipid core** — accumulation of oxidized LDL, cholesterol esters, and cellular debris; creates mechanical stress on the cap. 3. **Macrophage activation** — produces tissue factor (TF), which triggers thrombosis upon plaque rupture. 4. **Smooth muscle cell apoptosis** — reduces cap integrity; mediated by inflammatory cytokines (TNF-α, IL-1β). **High-Yield:** The **thin fibrous cap overlying a large lipid core** is the single best discriminator. This is the hallmark of vulnerability and the basis for plaque rupture in acute coronary syndrome. **Clinical Pearl:** Intravascular ultrasound (IVUS) and optical coherence tomography (OCT) can identify vulnerable plaques *in vivo* by measuring fibrous cap thickness and lipid pool size — increasingly used for risk stratification in acute coronary syndromes. [cite:Robbins 10e Ch 11]