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Old 04-22-2010
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Urine Sample Best Renal Investigation?

A 5-month-old male infant has a urine output of less than 0.1 mL/kg/hr shortly after undergoing major surgery. On examination, he has generalized edema. His blood pressure is 94/48 mm Hg, pulse is 140/min, and respirations are 20/min. His blood urea nitrogen is 38 mg/dL, and serum creatinine is 1.4 mg/dL. Initial urinalysis shows a specific gravity of 1.018 and 2+ protein. Microscopic examination of the urine sample reveals 1 WBC per high-power field (HPF), 18 RBCs per HPF, and 5 granular casts per HPF. His fractional excretion of sodium is 3.2 %. Which of the following is the most appropriate next step in diagnosis?
A. CT of the abdomen and pelvis
B. Cystourethrography
C. Intravenous pyelography
D. Renal biopsy
E. Renal ultrasonography
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Old 04-22-2010
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D. Renal biopsy
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D. Renal biopsy
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Old 04-26-2010
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Default The correct answer is E!

The correct answer is Ultrasound

This infant developed acute renal failure (ARF) in the immediate post-op period, as manifested by the increase in blood urea nitrogen and serum creatinine and the decrease in urine output. ARF can be classified into prerenal, renal, and postrenal. Prerenal causes include hypovolemia secondary to severe dehydration, hemorrhage, and hypotension secondary to shock. Renal causes include acute tubular necrosis (ATN), parenchymal disorders (e.g., glomerulonephritis), and vascular disorders (e.g., renal artery thrombosis or renal vein thrombosis). ATN is characterized by mild proteinuria, microscopic hematuria, and the presence of coarse granular casts in the urine. A fractional excretion of sodium greater than 2% (or 2.5% in neonates) is consistent with renal causes of ARF.

Renal ultrasonography is the imaging study of choice for this patient because it provides both anatomic and structural information about the kidneys. The study is noninvasive and can be easily done by the bedside.

CT of the abdomen and pelvis (choice A) can provide more anatomic details but is not a good initial imaging study, especially in this case. The contrast dye needed for CT can cause further damage to the kidneys and thus worsen renal failure. The same argument applies to intravenous pyelography (choice C).

Cystourethrography (choice B) provides structural details of the urinary bladder and the urethra, but it is not indicated in this case.

Renal biopsy (choice D) is the gold standard of diagnosing renal disease, but it is not indicated as an initial study. It might be useful in prolonged renal failure with an unidentified cause.
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