Concept:The urea concentration is lowest where the filtrate first enters the tubule, before water is removed.
Explanation:Glomerular filtrate enters the tubule with a low, uniform concentration of urea.
As it moves along the tubule, most water is reabsorbed into the blood.
Urea is not reabsorbed to the same extent as water.
Therefore, the urea becomes more concentrated in the later parts of the tubule.
The earliest part, labelled I, contains the lowest concentration of urea.
Answer:A. I
