Question

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what is actively transported back into the bloodstream by the proximal convoluted tubule?
calcium ions.
sodium ions.
potassium ions.
magnesium ions.
chloride ions.

Explanation:

The proximal convoluted tubule is an important part of the nephron in the kidney. Sodium ions ($Na^+$) are actively transported back into the bloodstream here. This active transport is crucial for reabsorption processes in the kidney. Calcium ions ($Ca^{2+}$) reabsorption is more regulated by hormones like parathyroid hormone and mainly occurs in different parts of the nephron (e.g., thick ascending limb in some cases). Potassium ions ($K^+$) secretion and reabsorption have complex regulation (e.g., in the distal convoluted tubule and collecting duct). Magnesium ions ($Mg^{2+}$) reabsorption is more significant in the loop of Henle. Chloride ions ($Cl^-$) often follow sodium ions passively in many cases (e.g., through paracellular transport in some regions).

Answer:

Sodium ions.