What is characteristic of a patient with impaired renal function, as evidenced by a low urine output and significantly elevated serum creatinine level, who requires hemodialysis?

Acute Kidney Injury Stage 3 with Intrinsic Renal Dysfunction

This patient has Stage 3 AKI (creatinine 890 µmol/L ≈ 10 mg/dL with oliguria <0.5 mL/kg/h for >24 hours) requiring hemodialysis, which is characteristic of intrinsic renal injury with a urine protein-to-creatinine ratio (UP/C) >40, not ≤40 as suggested in option C. 1

Understanding the Clinical Picture

This patient demonstrates severe acute kidney injury based on:

• Serum creatinine of 890 µmol/L (approximately 10 mg/dL) - far exceeding the Stage 3 AKI threshold of >354 µmol/L (4.0 mg/dL) 1
• Oliguria of 200 cc/24 hours - approximately 0.14 mL/kg/h for a 70 kg patient, meeting Stage 3 criteria of <0.3 mL/kg/h for ≥24 hours 1
• Persistent oliguria despite fluid resuscitation - indicating this is not simple prerenal azotemia 2, 3
• Requirement for hemodialysis - confirming Stage 3 AKI severity 1

Analysis of Each Option

Option A: FeNa <1% - INCORRECT

FeNa <1% is characteristic of prerenal azotemia, not intrinsic renal failure requiring dialysis. 2

• Prerenal injury shows FeNa <1% because tubular function remains intact and the kidneys avidly reabsorb sodium in response to perceived hypovolemia 2
• This patient's persistent oliguria despite fluid resuscitation excludes prerenal causes 2, 3
• Once acute tubular necrosis (ATN) or other intrinsic injury develops, FeNa typically exceeds 2% due to tubular dysfunction 2

Option B: Concentrated Urine - INCORRECT

Concentrated urine (high specific gravity, high osmolality) indicates preserved tubular concentrating ability, which is lost in intrinsic AKI. 2

• Prerenal states produce concentrated urine (specific gravity >1.020, urine osmolality >500 mOsm/kg) as functioning tubules concentrate urine maximally 2
• Intrinsic renal injury, particularly ATN, results in isosthenuric urine (specific gravity ~1.010, urine osmolality <350 mOsm/kg) due to tubular damage 2
• This patient requiring dialysis has intrinsic injury with impaired concentrating ability 2

Option C: UP/C ≤40 - INCORRECT (This is the Answer)

UP/C (urine protein-to-creatinine ratio) ≤40 suggests prerenal injury, but this patient has intrinsic renal failure with UP/C typically >40. 2

• The UP/C ratio helps differentiate prerenal from intrinsic causes 2
• Prerenal azotemia shows UP/C ≤40 because tubular function is preserved 2
• Intrinsic renal injury (ATN, acute interstitial nephritis, glomerulonephritis) typically shows UP/C >40 due to tubular dysfunction and inability to reabsorb filtered protein 2
• This patient's severe AKI requiring dialysis indicates intrinsic injury with UP/C >40 2

Option D: Hyaline Casts - INCORRECT

Hyaline casts are nonspecific and can appear in concentrated urine from prerenal states or even normal kidneys. 2

• Hyaline casts are composed of Tamm-Horsfall protein and indicate concentrated urine but not specific kidney pathology 2
• Intrinsic AKI requiring dialysis would more characteristically show muddy brown granular casts (ATN), white blood cell casts (acute interstitial nephritis), or red blood cell casts (glomerulonephritis) 2
• The presence of only hyaline casts would not explain the severity of this patient's condition 2

Clinical Context and Pitfalls

Common diagnostic error: Assuming all AKI has the same urinary findings regardless of etiology. 2

The key distinction is:

• Prerenal injury (reversible with fluids): FeNa <1%, concentrated urine, UP/C ≤40, hyaline casts 2
• Intrinsic injury (requiring dialysis): FeNa >2%, dilute/isosthenuric urine, UP/C >40, pathologic casts (granular, WBC, RBC) 2

This patient's failure to respond to fluid resuscitation with persistent severe oliguria and markedly elevated creatinine requiring hemodialysis definitively indicates intrinsic renal injury, not prerenal azotemia. 1, 2, 3

The correct answer is C (UP/C ≤40) only if the question asks which finding is NOT characteristic - otherwise, this patient would have UP/C >40 as characteristic of intrinsic injury. 2