**Core Concept**
The primary change occurring in blood vessels in hypertension is an increase in vascular resistance due to structural modifications. This is primarily attributed to the remodeling of small arteries and arterioles, which is characterized by thickening of the vessel wall and a decrease in lumen diameter.

**Why the Correct Answer is Right**
The thickening of the blood vessel wall in hypertension is primarily due to an increase in the number of smooth muscle cells and the deposition of extracellular matrix, such as collagen and elastin. This remodeling process is mediated by various factors, including angiotensin II, endothelin-1, and aldosterone, which stimulate the proliferation and migration of smooth muscle cells. The resulting increase in vascular resistance impairs blood flow and contributes to the development of hypertension.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the primary change in blood vessels in hypertension is not a decrease in vascular resistance, but rather an increase due to structural remodeling.
**Option B:** This option is incorrect because while inflammation may play a role in the development of hypertension, it is not the primary change occurring in blood vessels.
**Option C:** This option is incorrect because the primary change in blood vessels in hypertension is not a decrease in blood vessel wall thickness, but rather an increase due to remodeling.

**Clinical Pearl / High-Yield Fact**
The remodeling of small arteries and arterioles in hypertension can be reversed with antihypertensive therapy, particularly with angiotensin-converting enzyme inhibitors (ACE inhibitors) and angiotensin receptor blockers (ARBs), which can help to reduce vascular resistance and improve blood flow.

**Correct Answer: D. Thickening of the blood vessel wall**