**Question:** GFR increases if:

A. Efferent arteriole constricts
B. Efferent arteriole dilates
C. Afferent arteriole constricts
D. Afferent arteriole dilates

**Core Concept:** Glomerular Filtration Rate (GFR) is the rate at which blood passes through the glomerulus of a kidney and the filtered fluid exits into the Bowman's capsule. GFR is influenced by the balance between the glomerular capillary pressure and the hydrostatic pressure in the afferent and efferent arterioles.

**Why the Correct Answer is Right:** GFR is influenced by the balance between the glomerular capillary pressure and the hydrostatic pressure in the afferent and efferent arterioles. Efferent arteriole dilates, which increases the glomerular capillary pressure, ultimately leading to an increased GFR.

**Why Each Wrong Option is Incorrect:**

A. Efferent arteriole constricts: This would decrease the glomerular capillary pressure, resulting in a reduced GFR.
B. Efferent arteriole dilates: This option is correct, as explained above.
C. Afferent arteriole constricts: This would decrease the glomerular capillary pressure, leading to a reduced GFR.
D. Afferent arteriole dilates: Similar to option A, this would decrease the glomerular capillary pressure and result in a reduced GFR.

**Clinical Pearl:** Understanding the regulation of GFR is crucial for assessing kidney function and diagnosing diseases affecting the nephrons. Knowledge of the afferent and efferent arterioles and their roles in GFR regulation helps physicians in diagnosing glomerular diseases like nephrosclerosis, diabetic nephropathy, and focal segmental glomerulosclerosis, among other conditions affecting kidney function.