Question

The components responsible for the counter-current mechanism in the kidney include all of the following EXCEPT:

• A. Sodium outflow in thick ascending limb
• B. Water outflow in thin descending limb
• C. Sodium outflow in thin ascending limb
• D. Flow of tubular fluid from PCT to DCT

Hint:

Which limb does the active sodium pumping - thick or thin? Pick the EXCEPT.

Answer 1

The Correct Option is C

Step 1: This is a negative-lead question - find the option that is NOT part of the counter-current mechanism. The counter-current system uses the loop of Henle (multiplier) and the vasa recta (exchanger) to build and preserve the medullary osmotic gradient.
Step 2: The valid components are: active sodium reabsorption (with potassium and chloride cotransport) out of the thick ascending limb, water reabsorption out of the water-permeable thin descending limb, urea recycling from the medullary collecting ducts, and the directional flow of tubular fluid through the loop.
Step 3: The key point is that the thick ascending limb actively pumps out sodium, but the thin ascending limb is the site of passive movements and is essentially impermeable to water; sodium leaves the thin ascending limb passively only as part of the overall scheme - so describing 'sodium outflow in thin ascending limb' as a defining counter-current component is the odd one out. Option (c) is the exception, hence the answer.
Step 4: The remaining options are all genuine elements of the mechanism: sodium leaves the thick ascending limb (a), water leaves the thin descending limb (b), and tubular fluid must flow onward from PCT toward DCT (d) for the multiplier to operate.