In humans, excretion of hypertonic urine is associated with the

Analyze the question: Hypertonic urine is urine that is more concentrated than the blood plasma. The question asks which part of the nephron is responsible for making this possible.

Explain the concept: The kidney can produce concentrated urine because of the countercurrent mechanism in the loop of Henle:

• The descending limb is permeable to water, so water leaves into the surrounding tissue.
• The ascending limb actively pumps out salts (Na⁺ and Cl⁻), so the medulla becomes very salty.
• This creates a high salt concentration gradient in the medulla.
• When ADH is present, the collecting duct becomes permeable to water. As it passes through the salty medulla, water is drawn out by osmosis, leaving the urine concentrated (hypertonic).

Step-by-step check of the options:

• A. Distal convoluted tubule: Fine-tunes salt balance and pH, and responds to ADH and aldosterone. It does not create the medullary gradient.
• B. Loop of Henle: Creates the concentration gradient in the medulla that makes hypertonic urine possible.
• C. Bowman's capsule: Site of ultrafiltration only. The filtrate has the same concentration as plasma (without large proteins).
• D. Proximal convoluted tubule: Reabsorbs about 65-70% of the filtrate, including glucose and amino acids. The fluid remains isotonic here.

Answer: B. Loop of Henle

Exam tip: Link the words "Loop = Longer = Concentrated". Mammals with long loops of Henle (such as desert rats) produce very concentrated urine. Whenever you see "hypertonic urine" or "water conservation," think loop of Henle.