A 65-year-old man undergoes emergency repair of a ruptured abdominal aortic aneurysm. Intraoperatively, his mean arterial pressure drops to 50 mmHg and remains <55 mmHg for 6 hours. Postoperatively, he develops oliguria with serum creatinine rising from 1.0 to 3.2 mg/dL within 48 hours. Urinalysis shows muddy-brown granular casts and renal tubular epithelial cells; urine sodium is 45 mmol/L and FENa is 3.5%. At autopsy, both kidneys show the gross appearance depicted in the diagram. The structure marked **A** (pale, swollen cortex) reflects ischemic injury predominantly affecting which tubular segment?
A. S3 segment of proximal tubule and medullary thick ascending limb (watershed zone with high oxygen demand)
B. Thin descending limb of loop of Henle (osmotic gradient maintenance)
C. S1 and S2 segments of proximal tubule (high metabolic activity)
D. Distal convoluted tubule and collecting duct (principal cells)
Explanation
Why S3 segment of proximal tubule and medullary thick ascending limb is right
The pale, swollen cortex marked A in ischemic acute tubular necrosis (ATN) reflects the gross pathology of the "shock kidney." The S3 segment of the proximal tubule and the medullary thick ascending limb are located in a critical watershed zone with the highest oxygen consumption in the kidney. When systemic hypotension drops renal perfusion below the autoregulatory threshold (MAP <55 mmHg), these segments suffer the most severe ischemic injury because they are perched between the cortical and medullary blood supplies and have minimal oxygen reserve. Tubular epithelial cells in these zones swell, lose brush borders, detach from basement membranes, and slough into the lumen, forming the muddy-brown granular casts seen in this patient's urine. This pattern is pathognomonic for ischemic ATN and is the classic histologic finding in shock kidney (Robbins Pathology; KDIGO AKI guidelines).
Why each distractor is wrong
S1 and S2 segments of proximal tubule: While these segments have high metabolic activity, they are located in the cortex and have better collateral blood supply than the S3 segment. Ischemic ATN preferentially spares S1–S2 and targets the more vulnerable S3 segment in the outer medulla.
Distal convoluted tubule and collecting duct: These segments are less metabolically active than the proximal tubule and are not the primary sites of injury in ischemic ATN. They are affected secondarily by cast obstruction and tubuloglomerular feedback, not by direct ischemic necrosis.
Thin descending limb of loop of Henle: This segment is permeable to water and solutes but has minimal metabolic demand and is not a primary target of ischemic injury in shock kidney. It is not responsible for the muddy-brown cast formation seen in this case.
High-YieldNEET PG
Ischemic ATN targets the S3 segment and medullary thick ascending limb because they sit in a watershed zone with the highest oxygen consumption and poorest collateral perfusion—remember "outer medulla is the Achilles heel of the kidney."
Robbins Pathology; KDIGO AKI guidelines
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