MEDizzy - In a healthy person with a normal balanced diet, which of

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Renal and Urinary Systems

In a healthy person with a normal balanced diet, which of the following substances has the greatest fraction excreted in the urine compared to plasma?

ExplanationThe ratio of urine to plasma concentration for paraaminohippurate (PAH) is about 585, compared to a ratio of 1 for a substance with the same plasma and urine concentration. The reason for the extensive increase in concentration is the fact that PAH is both filtered and secreted. In fact, it is so rapidly secreted by the proximal tubule that the PAH plasma concentration is nearly completely removed while flowing through the kidney and excreted in urine, which makes it a clinical estimate for renal plasma flow. (A, C) The urine-to-plasma ratio for creatinine and inulin are indeed high, since both become concentrated when 99% of water is reabsorbed while they remain in the tubular fluid, and in the case of creatinine a small amount is even secreted. However, the ratio is about 125 for inulin and 140 for creatinine, which is lower compared to PAH. (B) Glucose is completely reabsorbed in the proximal tubule, so its urine-toplasma ratio is close to zero.