Solution: Hormonal Regulation of Salt and Water Balance

The nephron reabsorbs and secretes water and solutes at multiple segments, but the site of hormonal fine-tuning of final salt and water balance is the Collecting Duct. Bulk, obligatory reabsorption occurs earlier in the nephron (PCT, Loop of Henle), whereas the collecting duct performs the variable, hormone-controlled adjustments that match urine composition to the body's actual needs.

Why Collecting Ducts is Correct

• Antidiuretic Hormone (ADH/Vasopressin): Acts on the principal cells of the collecting duct, inserting aquaporin-2 water channels into the luminal membrane. This increases water permeability and permits water reabsorption driven by the medullary osmotic gradient — the key mechanism controlling water balance.
• Aldosterone: Acts on principal cells of the collecting duct (and late DCT) to upregulate ENaC and Na⁺/K⁺-ATPase, increasing sodium reabsorption (and secondarily water) while promoting potassium and hydrogen ion secretion — the key mechanism controlling salt balance.
• Atrial Natriuretic Peptide (ANP): Also acts at the collecting duct to inhibit sodium reabsorption, promoting natriuresis when blood volume is high.

Because both the major salt-regulating hormone (aldosterone) and the major water-regulating hormone (ADH) exert their principal, regulated effects at the collecting duct, this segment is considered the site of hormonal regulation of overall salt and water balance.

Why the Other Options Are Wrong

• Proximal Convoluted Tubule (PCT): Reabsorbs about 65-70% of filtered Na⁺ and water, but this is bulk, obligatory, isosmotic reabsorption that is largely independent of hormonal control (not the site of regulated fine-tuning).
• Distal Convoluted Tubule (DCT): Reabsorbs Na⁺ and Cl⁻ (via the NCC transporter, influenced modestly by aldosterone) but is impermeable to water and does not respond to ADH; it is not the principal site of water-balance regulation.
• Loop of Henle: Establishes the medullary hypertonic gradient (via the countercurrent multiplier) that makes water reabsorption possible downstream, but it does not itself carry out the hormone-regulated reabsorption of salt and water — it is a passive/active transport segment largely unresponsive to ADH or aldosterone.

Hence, the correct answer is Collecting Ducts (Option B), as this is where ADH and aldosterone exert their regulatory actions to finely control salt and water balance in the nephron.