WBR0404

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D. and Alison Leibowitz [1])]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathophysiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 72-year-old man is brought to the emergency department (ED) with high-grade fever and confusion. His temperature is 39.2 °C (102.5 °F), blood pressure is 78/58 mmHg, and heart rate is 108/min. Urinalysis in the ED demonstrates heavy leucocyturia and hematuria. The patient is diagnosed with urinary tract infection and septic shock and is admitted to the intensive care unit. The next day, the patient's foley catheter yields less than 100 mL of dark yellow urine over 24 hours. Further work-up demonstrates the values illustrated in the table shown below. What is the most likely cause of this patient’s oliguria? ]]
Answer A	AnswerA::Acute interstitial nephritis
Answer A Explanation	[[AnswerAExp::Acute interstitial nephritis, a type of renal injury, 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, a type of intrinsic renal injury, would not typically manifest with the lab values illustrated in the table. Diffuse bilateral renal cortical necrosis is usually an obstetric complication, which can also occur following trauma or sepsis.]]
Answer C	AnswerC::Renal stone complicated with hydronephrosis
Answer C Explanation	[[AnswerCExp::Renal stone complicated by hydronephrosis, a type of post-renal acute kidney injury, would generally manfest with elevated urinary sodium > 40 mEq/L.]]
Answer D	AnswerD::Urinary tract infection complicated by acute pyelonephritis
Answer D Explanation	[[AnswerDExp::WBC casts are characteristic of pyelonephritis. Because WBC casts are absent in this patient, the diagnosis of acute pyelonephritis is unlikely. In addition, acute pyelonephritis would not manifest with an elevated serum BUN/Creatinine ratio as demonstrated in this patient.]]
Answer E	AnswerE::Severe hypotension
Answer E Explanation	[[AnswerEExp::Hypotension may result in pre-renal acute kidney injury. Pre-renal azotemia is characterized by a serum BUN/Creatinine ratio > 20, Urinary osmolarity > 500 mOsm/kg, and urinary sodium < 20 mEq/L.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::Acute kidney injury (AKI), formerly known as acute renal failure, is characterized by an abrupt loss of kidney function resulting in a failure to excrete nitrogenous waste products (among others), and a disruption of fluid and electrolyte homeostasis. AKI defines a spectrum of disease with common clinical features including an increase in the serum creatinine and BUN levels, often associated with a reduction in urine volume. AKI can be caused by a multitude of factors broadly categorized into pre-renal (usually ischemic), intrinsic renal (usually toxic), and post-renal (usually obstructive) injuries. The patient’s presentation and lab values are characteristic of pre-renal acute kidney injury. Septic shock predisposes to acute kidney injury via ischemic, reperfusion, and direct inflammatory damage and is a common cause of pre-renal acute kidney 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 azotemia is more likely than other renal or post-renal etiologies. Educational Objective: Pre-renal azotemia, commonly resulting from severe hypotension, is a subtype of acute kidney injury characterized by a serum BUN/Creatinine ratio > 20, Urinary osmolarity > 500 mOsm/kg, and urinary sodium < 20 mEq/L. References: Klahr S, Miller SB. Acute oliguria. N Eng J Med. 1998;338:671-675]]
Approved	Approved::Yes
Keyword	WBRKeyword::kidney, WBRKeyword::excretory system, WBRKeyword::urine, WBRKeyword::renal, WBRKeyword::pre-renal azotemia, WBRKeyword::urinary sodium, WBRKeyword::renal injury, WBRKeyword::hypotension
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