WBR0389

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 38 year old male patient who is previously healthy is brought to the emergency department in sepsis. Patient does not smoke, does not drink alcohol, and does not take any medications. Initial management is promptly initiated. Several days later, the patient’s urine output becomes low. He is diagnosed with acute tubular necrosis (ATN). Which of the following statements is generally true about the difference between ATN and pre-renal injury?]]
Answer A	AnswerA::There is a prompt response to fluid challenge in pre-renal injury; response to fluid challenge is not rapid in ATN
Answer A Explanation	AnswerAExp::Prompt response to fluid challenge is characteristic of pre-renal kidney injury. ATN usually does not show prompt resolution of oliguria as seen in pre-renal injury.
Answer B	AnswerB::The ability to retain sodium in pre-renal injury is lost; whereas it is conserved in ATN
Answer B Explanation	AnswerBExp::The ability to retain sodium is lost in ATN. It is conversed in pre-renal injury
Answer C	AnswerC::Urine specific gravity in pre-renal injury is lower than urine specific gravity in ATN
Answer C Explanation	AnswerCExp::Pre-renal injury has a higher urine specific gravity than that in ATN
Answer D	AnswerD::In contrast to ATN, pre-renal injury is not a complication of medication intake
Answer D Explanation	[[AnswerDExp::Pre-renal injury could be a complication of medication intake. ACE-inhibitors-induced pre-renal injury is a classical case of medication-induced pre-renal injury. This is especially true in the case of renal artery stenosis in a solitary kidney or bilateral renal artery stenosis.]]
Answer E	AnswerE::Advanced age carries a worse prognosis in pre-renal injury, but not in ATN
Answer E Explanation	AnswerEExp::Due to impaired renal physiology with age, advanced age generally carries a worse prognosis for both ATN and pre-renal kidney injury.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Pre-renal kidney injury is a type of AKI characterized by renal hypoperfusion that cause impaired renal function. Pre-renal injury is generally reversed by appropriate fluid intake. Prompt response to fluid challenge is characteristic of pre-renal kidney injury. Since pre-renal injury is not a disorder of the kidney itself, the kidney’s ability to retain sodium in pre-renal injury is thus retained. As such, urine specific gravity in pre-renal injury is elevated. In contrast, acute tubular necrosis (ATN) is an intrinsic injury to the kidney. It may be a complication of prolonged pre-renal injury. ATN is the differential diagnosis of pre-renal injury. Distinguishing between the two is important for appropriate management. ATN usually does not show prompt resolution of oliguria as seen in pre-renal injury. ATN follows characteristic phases: inciting phase, maintenance phase where GFR is at its nadir, and finally recovery phase where oliguria is resolved and kidney function is restored, if the damage is not prolonged to cause permanent damage. Educational Objective: ATN should always be a differential diagnosis of pre-renal kidney injury. Pre-renal injury usually corrects promptly with adequate fluid challenge. This prompt response is not typically seen in ATN. Reference: Andreucci VE, Fuiano G, Russo D, et al. Vasomotor nephropathy in the elderly. Nephrol Dial Transplant. 1998;13(Suppl 7):17-24 Educational Objective: References: ]]
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