Which of the following substances are almost always reabsorbed in the kidneys?

In the kidneys, reabsorption is a crucial process that allows the body to reclaim essential substances that were filtered out during the process of urine formation. Among the substances that are typically reabsorbed are amino acids, glucose, and vitamins. These substances are vital for the body; therefore, the kidneys efficiently reabsorb them to prevent their loss in urine.

Amino acids and glucose are primarily reabsorbed in the proximal convoluted tubule through specific transporter proteins. This ensures that these important nutrients are returned to the bloodstream. Vitamins, particularly water-soluble ones, are also reabsorbed in varying degrees, depending on their concentration and the body's needs.

The other listed substances, while they may have some degree of reabsorption, do not have the same level of certainty. Urea, for example, is partially reabsorbed, but it is also excreted, which means it is not "almost always" reabsorbed. Water, sodium, and chloride do undergo significant reabsorption, but the distinguishing factor for the correct answer lies in the absolute necessity of maintaining levels of amino acids, glucose, and vitamins for metabolic functions, which makes option C the most appropriate.

Phosphates, potassium, and creatinine,