What is the fundamental histological difference between a true aneurysm and a pseudoaneurysm?

• A. True aneurysm is a contained rupture with a hematoma.
• B. Pseudoaneurysm requires dilation of all layers.
• C. True aneurysm involves dilation of all three layers; pseudoaneurysm is a contained rupture with a hematoma communicating with the arterial lumen.
• D. Pseudoaneurysm is a dilation of all three layers.

Answer: (C)

At the heart of this distinction is the structure of the vessel wall. A true aneurysm involves dilation where the entire arterial wall expands but remains intact, meaning all three layers—the intima, media, and adventitia—are present in the dilated wall, even though they may be stretched or degenerated. In contrast, a pseudoaneurysm results from a breach or rupture of the arterial wall; blood escapes into surrounding tissues and is contained there by a fibrous capsule or hematoma rather than by the original vessel wall. This capsule communicates with the arterial lumen, so the boundary of the pseudoaneurysm is not composed of the normal three arterial layers but of surrounding connective tissue and organized clot. Hence, the true aneurysm shows dilation of all three wall layers, while the pseudoaneurysm is a contained rupture with a hematoma that communicates with the artery.