WBR0404

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathophysiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 56 year old Caucasian male patient presents to the emergency department for fever. During his admission, the patient’s urine output is significantly decreased. Work-up reveals the values shown in the following table. Based on the values below, what is the most likely cause of the patient’s oliguria? ]]
Answer A	AnswerA::Acute interstitial nephritis
Answer A Explanation	[[AnswerAExp::Acute interstitial nephritis is a type of renal injury that would not manifest with high BUN/Creatinine ratio. In contrast, renal etiologies of acute kidney injury would have a serum BUN/Creatinine ratio < 15 with urine osmolarity commonly < 350 mOsm/kg.]]
Answer B	AnswerB::Bilateral renal cortical necrosis
Answer B Explanation	[[AnswerBExp::Diffuse bilateral renal cortical necrosis is also a type of intrinsic renal injury that would not typically contain the lab values in the vignette table. Diffuse bilateral renal cortical necrosis is usually an obstetric complication. It can also occur following trauma or sepsis.]]
Answer C	AnswerC::Renal stone complicated with hydronephrosis
Answer C Explanation	[[AnswerCExp::Renal stone complicated by hydronephrosis is a type of post-renal acute kidney injury that would generally have elevated urinary sodium > 40 mEq/L.]]
Answer D	AnswerD::Urinary tract infection complicated by acute pyelonephritis
Answer D Explanation	[[AnswerDExp::The hallmark of pyelonephritis is WBC casts, which are absent in this patient, making the diagnosis of acute pyelonephritis less likely. In addition, acute pyelonephritis would not have an elevated serum BUN/Creatinine ratio as seen in this patient.]]
Answer E	AnswerE::Severe hypotension
Answer E Explanation	[[AnswerEExp::hypotension may give rise to pre-renal acute kidney injury. Pre-renal injury is characterized by serum BUN/Creatinine ratio > 20, Urinary osmolarity > 500 mOsm/kg, and urinary sodium < 20 mEq/L.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::The patient’s presentation and lab values are in favor of a pre-renal injury. Among the list of options, severe hypotension is the only cause of pre-renal injury. To differentiate different types of acute kidney injury, calculation of BUN/Creatinine ratio is helpful. BUN/Creatinine = 80 / 2.5 = 32. Since the ratio is greater than 20, accompanied by elevated urine osmolarity > 500 mOsm/kg and urinary sodium < 20 mEq/L, the diagnosis of pre-renal injury in this patient makes it more likely than other renal or post-renal etiologies. Educational Objective: Pre-renal injury, commonly due to severe hypotension, is a subtype of acute kidney injury characterized by serum BUN/Creatinine ratio > 20, Urinary osmolarity > 500 mOsm/kg, and urinary sodium < 20 mEq/L. Reference: Klahr S, Miller SB. Acute oliguria. N Eng J Med. 1998;338:671-675 Educational Objective: References: ]]
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