In a healthy young man, which substance has the greatest fraction excreted by the kidneys?

In a healthy young man, urea is the substance that has the greatest fraction excreted by the kidneys. The kidneys play a vital role in the excretion of waste products from the metabolism of proteins, and urea is a primary nitrogenous waste product formed from the breakdown of amino acids.

While glucose and sodium are typically reabsorbed efficiently by the renal tubules, under normal healthy conditions, the entire amount of glucose is usually reabsorbed until the renal threshold is reached. Sodium, although excreted, is also extensively reabsorbed, primarily in the proximal tubule and loop of Henle, meaning that a relatively small fraction is actually excreted compared to the total amount filtered.

Water is crucial for maintaining homeostasis and is regulated based on hydration status. The kidneys adjust water excretion based on the body's needs, which can vary significantly; thus, not all filtered water is necessarily excreted.

In contrast, urea is excreted in considerable amounts relative to its filtration and is a key component in the regulation of nitrogen balance. This process means that a larger fraction of urea is found in the urine compared to glucose, sodium, or water, making it the correct answer in terms of the substance with