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

Question

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

Accepted Answer

A. Presence of a large lipid core with a thin fibrous cap. ## 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]

Answer

B. Absence of foam cells and macrophage infiltration

Answer

C. Predominance of smooth muscle cells in the intima

Answer

D. Calcification and fibrosis with minimal lipid content

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

Explanation

Distinguishing Stable from Vulnerable Plaques

Vulnerable Plaque Characteristics

Structural Comparison

Pathophysiology of Vulnerability

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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