A freely filterable substance that is neither reabsorbed nor secreted has a renal aery concentration of 12 mg/mL and a renal vein concentration of 9 mg/mL. Which of the following is the filtration fraction?
**Core Concept**
The filtration fraction is the ratio of the glomerular filtration rate (GFR) to the renal plasma flow (RPF). It represents the proportion of plasma that is filtered through the glomeruli. To calculate the filtration fraction, we need to determine the GFR and RPF.
**Why the Correct Answer is Right**
To calculate the GFR, we can use the formula: GFR = (C_in - C_out) * RPF, where C_in is the concentration of the substance in the renal artery, C_out is the concentration of the substance in the renal vein, and RPF is the renal plasma flow. Given that the substance is neither reabsorbed nor secreted, its concentration in the urine will be the same as its concentration in the renal vein. Assuming a normal RPF, we can use the given concentrations to calculate the GFR.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the fact that the substance is not reabsorbed or secreted.
**Option B:** This option is incorrect because it does not provide a valid formula for calculating the filtration fraction.
**Option C:** This option is incorrect because it does not account for the difference in concentrations between the renal artery and renal vein.
**Clinical Pearl / High-Yield Fact**
The filtration fraction is an important parameter in assessing renal function, and it can be used to determine the GFR and RPF. A normal filtration fraction is around 0.2.
**Correct Answer:** C. (0.25)