Relaxation of mesangial cells is responsible for
**Core Concept**
The underlying principle being tested is the regulation of renal blood flow and glomerular filtration rate, which is influenced by the contraction and relaxation of **mesangial cells**. These cells play a crucial role in the **glomerulus**, affecting the surface area available for filtration.
**Why the Correct Answer is Right**
Relaxation of **mesangial cells** increases the surface area for filtration, thereby increasing the glomerular filtration rate (**GFR**). This is because when **mesangial cells** relax, they reduce the resistance in the glomerular capillaries, allowing more blood to flow through and increasing the amount of fluid being filtered.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because contraction, not relaxation, of **mesangial cells** would decrease the **GFR**.
**Option B:** Incorrect as it's unrelated to the function of **mesangial cells** in regulating **GFR**.
**Option C:** Incorrect because while **mesangial cells** do have a role in supporting the glomerular capillaries, their relaxation specifically affects **GFR**.
**Clinical Pearl / High-Yield Fact**
Remember, **mesangial cells** regulate **GFR** by adjusting the surface area for filtration through contraction and relaxation, making them crucial for maintaining proper kidney function.
**Correct Answer:** D. Increase in glomerular filtration rate