A 33-year-old woman has had increasing lethargy and decreased urine output for the past week. Laboratory studies show her serum creatinine is 4.3 mg/ dL and urea nitrogen 40 mg/dL. A renal biopsy is performed, and the specimen is examined using electron microscopy. Which of the following morphologic cellular changes most likely suggests a diagnosis of acute tubular necrosis?

**Core Concept**
Acute tubular necrosis (ATN) is a type of acute kidney injury characterized by the death of tubular cells in the kidneys, leading to impaired renal function. The pathogenesis of ATN involves ischemia, toxins, or other insults that cause tubular cell injury and necrosis.

**Why the Correct Answer is Right**
The correct answer is related to the characteristic morphologic changes seen in ATN on electron microscopy. In ATN, the tubular cells undergo changes such as cell swelling, loss of microvilli, and the formation of "foot process fusion," which is the fusion of the foot processes of adjacent podocytes. This change is due to the disruption of the normal interdigitating pattern of podocyte foot processes, which is essential for maintaining the glomerular filtration barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** The presence of mesangial interposition, which is a feature of membranoproliferative glomerulonephritis, is not characteristic of ATN.

**Option B:** The formation of "humps" or "hump-like" structures within the glomerular basement membrane is not a feature of ATN.

**Option C:** The presence of fibrin caps on the glomeruli, which is a feature of fibrillary glomerulonephritis, is not characteristic of ATN.

**Clinical Pearl / High-Yield Fact**
ATN can be caused by various insults, including ischemia, toxins, and nephrotoxic medications. The diagnosis of ATN is often made based on clinical findings, laboratory studies, and renal biopsy results.

**Correct Answer: C.**

✓ Correct Answer: C. Nuclear fragmentation