**Question:** GFR is opposed by

A. renal vasoconstriction
B. tubular reabsorption
C. sodium and water reabsorption
D. sympathetic stimulation

**Core Concept:** Glomerular filtration rate (GFR) is the rate at which blood plasma is filtered by the glomerulus in the nephron, which is an important indicator of kidney function. It is the sum of fluid, ions, glucose, amino acids, and small lipid-soluble molecules being filtered in the nephron.

**Why the Correct Answer is Right:** GFR is opposed by tubular reabsorption because the kidneys reabsorb essential substances like water, glucose, ions, and proteins from the filtrate back into the bloodstream, maintaining homeostasis and preventing excess waste products from entering the urine. This reabsorption process helps maintain essential substances in the body while eliminating waste products.

**Why Each Wrong Option is Incorrect:**

A. Renal vasoconstriction: Although renal vasoconstriction can decrease GFR temporarily by narrowing the afferent arteriole, it is not the primary mechanism opposing GFR. Tubular reabsorption plays a more significant role in regulating GFR.

B. Tubular reabsorption: As mentioned earlier, tubular reabsorption opposes GFR by taking essential substances back into the bloodstream.

C. Sodium and water reabsorption: While sodium and water reabsorption are essential components of tubular reabsorption, the answer is broader because not all filtered substances are sodium and water. The correct answer includes all substances reabsorbed from the filtrate back into the bloodstream.

D. Sympathetic stimulation: Although sympathetic stimulation can affect GFR by inducing vasoconstriction, it is not the primary mechanism opposing GFR. Tubular reabsorption plays a more significant role in regulating GFR.

**Clinical Pearl:** Tubular reabsorption is a crucial process in maintaining homeostasis, as it ensures the essential substances are returned to the bloodstream while waste products are eliminated. Understanding the various reabsorption mechanisms in the nephron is crucial for evaluating renal function and diagnosing kidney disorders.