**Core Concept**
The afferent arteriole to the kidney glomerulus plays a crucial role in regulating blood flow and pressure within the glomerulus, thereby influencing the glomerular filtration rate (GFR). The GFR is the rate at which fluid is filtered through the glomerulus into the Bowman's capsule.

**Why the Correct Answer is Right**
Constriction of the afferent arteriole to the kidney glomerulus reduces the cross-sectional area available for blood flow, leading to an increase in resistance and a subsequent decrease in blood pressure within the glomerulus. This reduction in glomerular capillary hydrostatic pressure results in a decrease in the net filtration pressure, which in turn reduces the glomerular filtration rate (GFR). The decreased GFR leads to a reduction in the amount of fluid filtered through the glomerulus into the Bowman's capsule, ultimately resulting in decreased urinary output.

**Why Each Wrong Option is Incorrect**
**Option B:** Increased constriction of the afferent arteriole would not lead to an increase in urine output; rather, it would result in decreased urine output due to the reduction in GFR.
**Option C:** Constriction of the afferent arteriole would not increase the GFR, as it reduces the cross-sectional area available for blood flow, leading to an increase in resistance and a subsequent decrease in blood pressure within the glomerulus.
**Option D:** While decreased urine output is a consequence of constricted afferent arteriole, option A provides a more accurate description of the underlying physiological mechanism.

**Clinical Pearl / High-Yield Fact**
The regulation of GFR is a critical mechanism for maintaining fluid balance and electrolyte homeostasis in the body. Understanding the effects of afferent arteriole constriction on GFR is essential for managing patients with conditions such as renal artery stenosis or renovascular hypertension.

**✓ Correct Answer: A. Reduction of glomerular filtration rate and decreased urinary output**