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)