An experimental drug selectively increases medullary blood flow through the vasa recta without altering GFR or solute delivery to the loop of Henle. What is the expected effect on maximal urinary concentrating ability?
- A Concentrating ability improves because oxygen delivery to the inner medulla rises
- B Concentrating ability falls because the medullary osmotic gradient is washed out ✓
- C No effect occurs because the vasa recta function solely as nutrient vessels
- D Concentrating ability rises because urea recycling through the vasa recta accelerates
Correct answer: B. Concentrating ability falls because the medullary osmotic gradient is washed out
Explanation
The vasa recta act as countercurrent exchangers: they passively take up solute from the interstitium in one limb and return it in the other, preserving the corticomedullary gradient while supplying blood. When medullary flow increases, this exchange becomes inefficient, solute is carried away, and the hypertonic inner medulla is diluted. The result is a lower achievable urine osmolality, independent of ADH levels or loop function.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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