**Core Concept**
Glomerular filtration rate (GFR) is the rate at which the kidneys filter waste and excess fluids from the blood. It is an important indicator of kidney function, and several factors can influence its value.

**Why the Correct Answer is Right**
The GFR decreases in various conditions that affect kidney function, including decreased blood volume (hypovolemia), increased blood pressure (hypertension), and diseases that damage the kidneys, such as diabetic nephropathy or chronic kidney disease. The GFR can also decrease in conditions that affect the blood vessels, such as atherosclerosis or vasculitis, which can reduce blood flow to the kidneys. Additionally, the GFR can decrease in response to certain medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs) that can reduce blood flow to the kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly affect GFR. While certain medications may have a secondary effect on GFR, this option is too vague to be a correct answer.

**Option B:** This option is incorrect because it actually increases GFR. Certain medications, such as loop diuretics, can increase GFR by reducing blood pressure and increasing blood flow to the kidneys.

**Option C:** This option is incorrect because it does not directly affect GFR. While certain conditions may affect GFR indirectly, this option is too vague to be a correct answer.

**Option D:** This option is incorrect because it actually increases GFR. Certain conditions, such as pregnancy, can increase GFR by increasing blood flow to the kidneys.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to confuse GFR with other kidney function tests, such as serum creatinine or urea levels. Remember that GFR is a dynamic measure of kidney function that can change in response to various conditions, whereas serum creatinine and urea levels are more static measures.

**Correct Answer:** D.