**Core Concept**
Pemberton's sign is a clinical test used to assess the integrity of the left subclavian artery. It is an indirect measure of the blood flow through the left subclavian artery, which is a common site for atherosclerotic disease.

**Why the Correct Answer is Right**
Pemberton's sign is elicited by having the patient raise their arms above their head. When the left arm is raised, the blood flow through the left subclavian artery is reduced, which can cause a decrease in the blood pressure in the left arm compared to the right arm. This is due to the fact that the left subclavian artery is often affected by atherosclerotic disease, leading to a reduction in its diameter and subsequent decrease in blood flow.

**Why Each Wrong Option is Incorrect**
**Option A:** Swallowing is not related to Pemberton's sign, which is a test of subclavian artery blood flow.
**Option C:** Extrusion of tongue is a feature of a different clinical sign, such as the "tongue protrusion test" used to assess hypoglossal nerve function.
**Option D:** Extension of neck is not a part of Pemberton's sign, which specifically involves raising the arms above the head.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to confuse Pemberton's sign with the "Barber Forbes sign", which is used to assess the left subclavian artery but involves the patient clenching their fist and extending their arm, rather than raising their arm above their head.

**✓ Correct Answer: B. Raising of arms**