Question

As a result of the non-selectivity of the kidney's filtration of small molecules, _____. see concept 44.4 (page 986) as a result of the non-selectivity of the kidney's filtration of small molecules, _____. see concept 44.4 (page 986) the kidneys have little control over body fluid composition useful substances must be selectively reabsorbed the proportions of all the substances in the blood are the same as in the urine many useful substances are lost in the urine urine is always much less concentrated than blood

Answer 1

useful substances must be selectively reabsorbed

Filtration of blood by kidney only selected based on the molecule size. That means all small molecule will enter the kidney, doesn't matter whether it is toxic or useful.
But kidney has another mechanism such as reabsorption. In reabsorption, some specific useful molecule(like glucose) will be absorbed back.