How does Antidiuretic Hormone (ADH / Vasopressin), released from the posterior pituitary, promote the formation of concentrated urine?

Answer and explanation

Correct answer: C. Inserts aquaporin-2 water channels in DCT and collecting duct

  1. Step 1: Hypothalamic osmoreceptors detect dehydration or increased plasma osmolarity (>300 mOsm/L) and stimulate the neurohypophysis to release ADH.
  2. Step 2: ADH binds to vasopressin receptors on principal cells of the distal convoluted tubule and collecting duct.
  3. Step 3: This triggers intracellular cAMP signaling that mobilizes intracellular vesicles containing aquaporin-2 channels, inserting them into the apical luminal membrane.
  4. Step 4: Water is passively drawn out of the tubular fluid into the hyperosmotic medullary interstitium, conserving water and producing a small volume of hypertonic urine.
Answer
It inserts aquaporin-2 water channels into the apical membranes of DCT and collecting duct cells, increasing facultative water reabsorption
Why not the others: 'It blocks all sodium-potassium pumps...' would prevent urine concentration and cause massive fluid loss; 'It stimulates podocytes to contract...' confuses tubular water reabsorption with glomerular filtration; 'It degrades urea...' is chemically false.

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