**Core Concept**
Mycotic aneurysms are infections-induced arterial dilatations, typically arising from bacterial endocarditis or sepsis. They result from direct vascular wall invasion by organisms, leading to localized inflammation, necrosis, and vessel weakening. These aneurysms are distinct from atherosclerotic aneurysms in morphology and etiology.

**Why the Correct Answer is Right**
Mycotic aneurysms are typically **thin-walled, multilobed**, and have a **narrow neck** due to focal infection and wall destruction. They most commonly involve the **aorta and visceral arteries** (e.g., renal, mesenteric), not the systemic arteries. The **etiology is usually bacterial endocarditis**, especially in patients with prosthetic valves or underlying heart disease. The shape is typically **thimble-shaped or multilobulated**, not fusiform. Therefore, stating that it is "commonly fusiform" is incorrect.

**Why Each Wrong Option is Incorrect**
Option A: Bacterial endocarditis is a common etiology — this is **true** and a hallmark of mycotic aneurysms.
Option C: Aorta and visceral vessels are commonly affected — this is **accurate**, as these vessels are most frequently involved.
Option D: Multilobed aneurysm with narrow neck — this is a **classic feature** of mycotic aneurysms due to infectious destruction of vessel wall.

**Clinical Pearl / High-Yield Fact**
Mycotic aneurysms are often **multilobulated**, **narrow-necked**, and **associated with septic emboli**; they are life-threatening and require urgent surgical intervention. Always think "infection" when seeing a non-fusiform, infected aneurysm.

✓ Correct Answer: B. Commonly it is fusiform aneurysm