Chloride ion is reabsorbed in the thick ascending limb of the nephron primarily by which of the following mechanisms?

• A. Simple diffusion through the lipid bilayer
• B. Facilitated diffusion via aquaporins
• C. Chloride-bicarbonate exchanger
• D. Na\(^+\)-K\(^+\)-2Cl\(^-\) cotransporter

Answer: (D)

Verified step by step guidance

1. Understand the role of the thick ascending limb of the nephron: This region is responsible for reabsorbing ions like sodium (Na\(^+\)), potassium (K\(^+\)), and chloride (Cl\(^-\)) to maintain electrolyte balance and concentrate urine.
2. Recognize that chloride ions cannot passively diffuse through the lipid bilayer because they are charged particles. Charged ions require specific transport mechanisms to cross cell membranes.
3. Identify the Na\(^+\)-K\(^+\)-2Cl\(^-\) cotransporter as the primary mechanism for chloride reabsorption in the thick ascending limb. This cotransporter moves Na\(^+\), K\(^+\), and Cl\(^-\) ions together into the epithelial cells from the tubular fluid.
4. Understand the driving force behind this cotransporter: The sodium gradient created by the Na\(^+\)-K\(^+\) ATPase pump on the basolateral membrane provides the energy for the cotransporter to function.
5. Note that other mechanisms like aquaporins or chloride-bicarbonate exchangers are not involved in chloride reabsorption in this specific region of the nephron. Aquaporins facilitate water movement, and chloride-bicarbonate exchangers are used in other parts of the nephron for acid-base balance.