A 65-year-old man has urine output of 10 mL/h following abdominal aortic aneurysmectomy. Acute tubular necrosis is suggested by the presence of which of the following?
**Core Concept**
Acute tubular necrosis (ATN) is a form of acute kidney injury characterized by damage to the tubular epithelial cells of the kidneys, leading to a loss of their function. This condition often occurs in the setting of ischemia or nephrotoxicity.
**Why the Correct Answer is Right**
The presence of granular or muddy brown casts in the urine is a hallmark of ATN. These casts are formed when the damaged tubular cells break down and release their cellular contents, including proteins and cellular debris, into the tubular lumen. The combination of these cellular components with the tubular fluid creates the characteristic appearance of granular or muddy brown casts. This finding is specific to ATN and is not seen in other forms of acute kidney injury.
**Why Each Wrong Option is Incorrect**
**Option A:** BUN and creatinine levels may be elevated in ATN, but they are not specific markers of the condition. Other forms of acute kidney injury can also cause an increase in these values.
**Option B:** Hematuria may be present in ATN, but it is not a sensitive or specific marker for the condition. Hematuria can be caused by a variety of conditions, including urinary tract infections, kidney stones, and glomerulonephritis.
**Option C:** Proteinuria may be present in ATN, but it is not a specific marker for the condition. Proteinuria can be caused by a variety of conditions, including glomerulonephritis, diabetic nephropathy, and minimal change disease.
**Option D:** Pyelonephritis is an infection of the kidney, which can cause acute kidney injury. However, it is not a specific marker for ATN, and the presence of pyelonephritis would require additional diagnostic testing to confirm.
**Clinical Pearl / High-Yield Fact**
Remember that the presence of granular or muddy brown casts in the urine is a specific marker for ATN, and should prompt further investigation into the underlying cause of acute kidney injury.
**Correct Answer:** C.