**Question:** In spider nevi, dilatation of blood vessels is due to:

A. Vasodilation
B. Vascular ectasia
C. Capillary aneurysms
D. Vasomotor dysfunction

**Core Concept:** Spider nevi, also known as telangiectasia or small red blood vessels, are thin, dilated blood vessels that appear on the skin's surface. They can be caused by various factors such as hormonal changes, pregnancy, menopause, or oral contraceptive use. In this case, we will focus on the underlying mechanisms of dilatation.

**Why the Correct Answer is Right:** The correct answer, **D. Vasomotor dysfunction**, refers to the impaired control of blood vessel diameter, leading to their dilation. This can be caused by various factors like neurogenic or hormonal factors. Hormonal changes, as seen in pregnancy, menopause, or oral contraceptive use, can cause vasomotor dysfunction, leading to spider nevi.

**Why Each Wrong Option is Incorrect:**

A. **Vasodilation (A)** is the widening or relaxation of blood vessels, which is the opposite of what occurs in spider nevi. Vasodilation results in reduced blood vessel diameter, not dilation.

B. **Vascular ectasia (B)** refers to abnormally dilated blood vessels but doesn't specifically address the underlying cause of spider nevi.

C. **Capillary aneurysms (C)** are enlarged blood vessels that are not the same as spider nevi. Spider nevi are characterized by small, thin blood vessels on the skin surface.

**Clinical Pearl:** Spider nevi are easily recognizable and generally benign, but they can be an indicator of deeper vascular malformations, such as hemangiomas or lymphatic malformations. A thorough clinical history and examination, along with appropriate investigations, are essential in determining the underlying cause and management strategy.

**Correct Answer:** D. Vasomotor dysfunction