**Core Concept:** Spider nevi, also known as telangiectasias or small dilated blood vessels, are clinically observed skin lesions. These lesions are usually present at birth and are classified as benign vascular anomalies. They are further divided into two types: capillary and venous telangiectasias. Capillary telangiectasias are caused by dilation of capillaries, while venous telangiectasias are caused by dilation of small veins.

**Why the Correct Answer is Right:** Spider nevi are caused by dilatation of blood vessels, specifically capillaries. This is due to the underlying condition called venous stasis, which results from an imbalance in the production and absorption of blood within the capillaries leading to their dilation. The dilated capillaries appear as red or purple spots on the skin surface due to increased blood flow and decreased capillary filtration.

**Why Each Wrong Option is Incorrect:**

A. Spider nevi are not caused by dilatation of blood vessels, but rather by venous stasis.
B. Dilatation of blood vessels due to inflammation or injury is not related to spider nevi.
C. Spider nevi are not related to the dilatation of blood vessels in the dermis, but rather result from venous stasis in the superficial capillaries.
D. Spider nevi are not associated with the dilatation of blood vessels in the dermis, but rather result from venous stasis in the superficial capillaries.

**Clinical Pearl:** Dilatation of capillaries in spider nevi is a consequence of the underlying venous stasis, which can lead to other vascular disorders like venous ulcers or varicose veins if left untreated. Early diagnosis and treatment can prevent further complications and improve the patient's quality of life.