Outline a practical differential diagnosis approach to AKI in a hospitalized patient.

• A. Determine pre/intrinsic/postrenal causes by history (volume status, meds, obstruction), exam, labs (BUN/Cr, FENa, FeUrea), urinalysis, imaging (ultrasound), and consult nephrology as needed
• B. Only order a CT scan to evaluate kidneys
• C. Ignore patient history and rely on creatinine alone
• D. The sequence of tests is irrelevant to diagnosis

Answer: (A)

A structured, stepwise approach to distinguishing prerenal, intrinsic, and postrenal AKI is essential in hospitalized patients. Start with a careful history and exam focused on volume status, medication exposure (nephrotoxins, NSAIDs, ACE inhibitors, diuretics), recent contrast, and symptoms of obstruction. Then use labs to classify the injury: the BUN-to-creatinine ratio can point toward decreased perfusion, but it isn’t definitive on its own. Fractional excretion of sodium and fractional excretion of urea help separate prerenal from intrinsic AKI, though diuretics can blunt FENa and FeUrea, so interpret them in context. Urinalysis adds clues from the urine itself—granular casts or pigmented casts suggest acute tubular necrosis, eosinophils can hint at drug-induced interstitial nephritis, and protein or red cell casts point to glomerular problems. Imaging with renal ultrasound is a first-line tool to identify obstruction, assess kidney size, and detect hydronephrosis; reserve CT for specific indications and be mindful of contrast in AKI. If the diagnosis remains unclear or kidney function does not improve with initial management, involve nephrology. This approach ensures you consider all major etiologies, use data in a logical sequence, and guide timely treatment.