Describe the functional flow of filtrate through a nephron and the role of the proximal tubule, loop of Henle, distal tubule, and collecting duct.

• A. Filtration at the glomerulus; proximal tubule reabsorbs Na+, water, glucose; loop of Henle creates a medullary osmotic gradient; distal tubule fine-tunes Na+, K+, Ca2+ under hormones; collecting duct adjusts final Na+ and water reabsorption.
• B. Filtration occurs in the proximal tubule; loop of Henle reabsorbs glucose; distal tubule creates medullary gradient; collecting duct sorts urea.
• C. Blood enters nephron, immediate reabsorption of all solutes occurs in collecting duct; glomerulus not involved.
• D. Filtration at loop of Henle; proximal tubule secretes water exclusively; distal tubule does nothing.

Answer: (A)

How filtrate moves through the nephron and how each segment handles reabsorption and hormonal regulation is what this question is testing. The process starts with filtration at the glomerulus, where a plasma-like filtrate enters the tubular system. In the proximal tubule, the bulk of reabsorption occurs: Na+ and water are reabsorbed together, and nutrients such as glucose (and amino acids) are pulled back into the blood, so the filtrate becomes less concentrated as it leaves this segment. The loop of Henle then establishes a medullary osmotic gradient essential for concentrating urine. The descending limb is highly permeable to water, so water exits into the hyperosmotic medulla, while the ascending limb actively reabsorbs Na+, K+, and Cl− but is relatively impermeable to water, which helps create and maintain the gradient. Moving into the distal tubule, fine-tuning occurs: Na+ reabsorption is adjusted, along with reabsorption or secretion of K+ and Ca2+, under hormonal control (notably aldosterone and PTH). Finally, the collecting duct modulates the final reabsorption of Na+ and water, with water reabsorption governed by antidiuretic hormone, allowing the body to produce more or less concentrated urine. This sequence explains why the described roles match the nephron’s functional flow and how filtration, reabsorption, and hormonal regulation shape the final urine.