**Core Concept**
Angiofibromas, specifically juvenile nasopharyngeal angiofibromas (JNA), are highly vascular tumors that cause significant morbidity due to excessive bleeding. The underlying reason for this excessive bleeding is related to the histopathological characteristics of these tumors.

**Why the Correct Answer is Right**
The primary reason angiofibromas bleed excessively is because the blood vessels within these tumors lack a contractile component, specifically smooth muscle. This absence of smooth muscle in the blood vessels leads to fragile and non-compliant vessels that are prone to rupture and bleeding. The vessels are also rich in small, thin-walled arterioles and venules that contribute to the increased vascularity and tendency for bleeding. The lack of a contractile component also makes these vessels unresponsive to vasoconstrictive stimuli, further exacerbating the bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** The lack of a capsule in angiofibromas is not the primary reason for excessive bleeding. While the capsule may play a role in containing the tumor, the underlying issue lies in the histopathological characteristics of the tumor itself.

**Option C:** Angiofibromas do not have multiple sites of origin; they typically arise from the nasopharynx, specifically from the sphenopalatine foramen.

**Option D:** This option is incorrect because only one of the options (B) accurately describes the reason for excessive bleeding in angiofibromas.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that angiofibromas are highly vascular tumors, and their tendency for excessive bleeding can make surgical management challenging. Preoperative embolization and meticulous surgical technique are often required to minimize bleeding complications.

**✓ Correct Answer: B. Vessels lack a contractile component**