**Core Concept**
Raynaud's disease is a vasospastic disorder characterized by episodic vasospasm of the digital arteries, leading to reduced blood flow to the fingers and toes. This condition is often triggered by cold temperatures or emotional stress.

**Why the Correct Answer is Right**
When a patient with Raynaud's disease immerses his hand in cold water, the cold stimulus triggers an exaggerated vasoconstrictive response, causing the blood vessels to constrict further. This leads to a significant reduction in blood flow to the affected area, resulting in pallor, pain, and eventually, cyanosis. The cold-induced vasospasm is mediated by the release of vasoconstrictor substances, such as endothelin-1, and the activation of alpha-adrenergic receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because vasodilation is the opposite of what occurs in Raynaud's disease. The condition is characterized by vasospasm, not vasodilation.
**Option B:** This option is incorrect because ischemia is a consequence of the vasospasm, not the primary response to cold stimulation. Ischemia occurs as a result of reduced blood flow to the affected area.
**Option C:** This option is incorrect because decreased oxygenation is a consequence of the reduced blood flow, not the primary response to cold stimulation.

**Clinical Pearl / High-Yield Fact**
Raynaud's disease can be distinguished from other vasospastic disorders by its characteristic "white, blue, and red" color changes, which correspond to the phases of pallor, cyanosis, and reperfusion.

**Correct Answer:** C.