There were brown granular casts in the urine sediment. Which is the most likely cause of this complication?

A 65yo woman with DM, HTN and normal kidney function underwent a total right hip replacement. She had massive haemorrhage during the operation and was given 8 units of packed RBC. The blood pressure dropped to 60/40 mm Hg for about two hours before it was corrected with blood transfusion. Two days after the surgery the serum creatinine level rose to 4.2 mg/dl (normal <1.5 mg/dl), BUN was 50 mg/dl (normal 10-20 mg/dl) and potassium 5.0 mmol/L (normal 3.5-5.0 mmol/l). There were brown granular casts in the urine sediment. Which is the most likely cause of this complication?

a. Diabetic nephropathy
b. Malignant hypertension
c. Acute tubular necrosis
d. Interstitial nephritis

Ans. The key is C. Acute tubular necrosis. [Hypotension even for some minutes or few hours can readily lead to acute tubular necrosis which is evident here by uremia and further supported by brown granular cast in the urine sediment].