Two substances that can probably be used to determine filtration fraction are –
**Core Concept**
Filtration fraction is the ratio of the glomerular filtration rate (GFR) to the renal plasma flow (RPF), calculated as GFR/RPF. It is an important indicator of kidney function, reflecting the proportion of plasma filtered by the glomeruli. The two substances used to determine filtration fraction should have a relationship with glomerular filtration and renal plasma flow.
**Why the Correct Answer is Right**
Inulin is a substance that is freely filtered by the glomeruli and neither reabsorbed nor secreted by the renal tubules. Thus, it is an ideal substance to measure GFR. On the other hand, para-amino hippuric acid (PAH) is almost completely cleared from the plasma by the kidneys, and its clearance rate is equal to RPF. Therefore, by measuring the clearance rates of inulin and PAH, we can calculate the filtration fraction.
**Why Each Wrong Option is Incorrect**
**Option A:** Creatinine is also freely filtered by the glomeruli, but it is also secreted by the renal tubules to some extent, making it less ideal for measuring GFR.
**Option B:** Urea is not freely filtered by the glomeruli and is also reabsorbed by the renal tubules, making it a poor choice for measuring GFR.
**Option C:** Mannitol is freely filtered by the glomeruli, but it is also reabsorbed by the renal tubules, making it less ideal for measuring GFR.
**Clinical Pearl / High-Yield Fact**
Inulin clearance is considered the gold standard for measuring GFR, while PAH clearance is used to estimate RPF.
**Correct Answer: C. Inulin and para-amino hippuric acid.