A 71-year-old man has been admitted for an elective right total knee replacement. He suffers from a post-operative urinary tract infection and is given 5 days of gentamicin.


Following this, his condition deteriorates with worsening acute kidney injury and dark brown urine. His blood tests show:

||||
|---------------------------|:-------:|--------------------|
|Urea|14.1 mmol/L|2.5 - 7.8|
|Creatinine|678 µmol/L|60 - 120|
|Urinary Sodium (24hr)|350 mmol/24hrs|75 - 300|


Which of the following is the most likely cause of this patient's acute kidney injury?

Options:
explanation: Pre-renal uraemia is a very common cause of acute kidney injury in hospitals. The creatinine and urea can rise to these levels if the kidneys are very poorly perfused, either from shock or severe dehydration, although the kidneys would hold on to electrolytes with very concentrated urine. Urinary sodium would be <10mmol/L in this case, and so makes this a less likely cause for this patient's symptoms
name: Pre-renal uraemia triggered by sepsis
label: d
answer: no
explanation: Post-infectious glomerulonephritis can occur after almost any infection, but typically occurs 1-4 weeks following infection. Classically it presents following pharyngeal infection with Streptococcus pyogenes. While it can present with acute kidney injury and frank haematuria/dark brown urine, it would usually also present with generalised oedema and hypertension due to salt retention
name: Post-infectious glomerulonephritis
label: e
answer: no
explanation: This patient has had an infection followed by 5 days of intravenous gentamicin - a nephrotoxic medication. Worsening renal function with dark brown urine should raise suspicion of acute tubular necrosis. This is confirmed with a raised urinary sodium, implying less tubular modification of the glomerular filtrate with increased urinary electrolyte losses
name: Acute tubular necrosis
label: a
answer: yes
explanation: Renal vein thrombosis can rarely occur following deep vein thrombosis in the legs - a common complication following knee surgery. It would unlikely present in an otherwise well adult unless there were severe dehydration or increased hypercoagulable state. If it were to cause such a severe acute kidney injury, it would present with haematuria and flank pain - opposed to dark brown urine with raised urinary sodium
name: Renal vein thrombosis
label: c
answer: no
explanation: Acute interstitial nephritis (AIN) is most commonly caused by medications (70-75% of cases), predominantly beta-lactam antibiotics, NSAIDs, rifampicin, ciprofloxacin, and diuretics. Classically patients with drug-induced AIN present with rash, pyrexia, and eosinophilia. They would also unlikely present with dark brown urine and more often present with urinary sediment of white cells, red cells, and white cell casts. Gross haematuria would also be distinctly unusual in AIN
name: Acute interstitial nephritis
label: b
answer: no

Correct answer: a


Learning point: Acute tubular necrosis can occur following the use of nephrotoxic medications such as certain antibiotics, contrast agents, or chemotherapy drugs, leading to impaired kidney function, elevated creatinine levels, and dark brown urine due to the presence of damaged tubular cells and myoglobin.