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Renal · USMLE Step 1

USMLE Renal: Tubular Physiology and Diuretic Mechanisms

Nephron segment transport function paired with the diuretic class that acts there and its mechanism.

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Which nephron segment reabsorbs about 65-70% of the filtered sodium and water?
The proximal convoluted tubule (PCT).
What transporter in the early proximal tubule reabsorbs filtered glucose?
SGLT2 (sodium-glucose cotransporter 2).
Which diuretic class acts in the proximal convoluted tubule?
Carbonic anhydrase inhibitors.
What is the mechanism of carbonic anhydrase inhibitors in the PCT?
They inhibit carbonic anhydrase, reducing bicarbonate reabsorption.
What acid-base disturbance results from carbonic anhydrase inhibitor use?
Metabolic acidosis.
What ophthalmic condition is acetazolamide commonly used to treat?
Glaucoma.
What condition can acetazolamide help prevent at high elevation?
Altitude sickness.
What is the mechanism of osmotic diuretics like mannitol?
They increase tubular fluid osmolarity, limiting passive water reabsorption, mainly in the proximal tubule and descending limb.
What is a major clinical use of mannitol?
Treating cerebral edema or elevated intracranial or intraocular pressure.
Which segment of the loop of Henle is highly permeable to water?
The thin descending limb.
Which segment of the loop of Henle is permeable to NaCl but impermeable to water?
The thin ascending limb.
What transporter sits on the luminal membrane of the thick ascending limb (TAL)?
The Na-K-2Cl cotransporter (NKCC2).
Is the thick ascending limb permeable to water?
No, it is impermeable to water.
Which diuretic class inhibits the NKCC2 cotransporter in the thick ascending limb?
Loop diuretics, such as furosemide.
Which loop diuretic is preferred in patients with a sulfa allergy?
Ethacrynic acid.

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