Final answer:
The proximal convoluted tubule is indicated by letter A. It reabsorbs ions, water, and nutrients, removes toxins, and adjusts filtrate pH.
Step-by-step explanation:
The letter that indicates the proximal convoluted tubule is A.
The proximal convoluted tubule is a part of the nephron in the kidney, responsible for reabsorbing ions, water, and nutrients from the filtrate, as well as removing toxins and adjusting the filtrate's pH.
For example, the cells of the proximal convoluted tubule actively transport ions across their membranes and possess a high concentration of mitochondria to produce sufficient ATP.