Q: Which compound is used to evaluate the function of the kidney specifically at the level of the glomerulus?  

Answer: B

The glomerular filtration rate (GFR) is a key indicator of renal function. Insulin is a low molecular weight protein that is freely filtered at the glomerulus, and not secreted or reabsorbed by the renal tubules. This property makes insulin an ideal marker to assess GFR. The insulin clearance test measures the rate at which insulin is cleared from the plasma by the kidneys, and is used to calculate GFR. In contrast, creatinine is a breakdown product of creatine phosphate in muscle tissue, and is produced at a relatively constant rate. Creatinine is not an ideal marker for assessing GFR, because a significant amount of it is secreted by the renal tubules, leading to an overestimation of GFR. Similarly, para-aminohippuric acid (PAH) is an organic anion that is completely extracted from the blood by the kidneys, and is used to measure renal plasma flow. PAH is not a good marker for GFR, because it is both filtered and secreted by the renal tubules. Insulin is an accurate marker for assessing GFR, and can be used to diagnose and monitor renal disease.

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