**Core Concept**
Oliguric acute tubular necrosis (ATN) is a clinical condition characterized by a significant reduction in renal blood flow, leading to ischemic damage to the renal tubules. This condition is often seen in patients with severe hypovolemia, shock, or nephrotoxicity.

**Why the Correct Answer is Right**
The oliguric phase of ATN is characterized by a marked decrease in urine output, typically less than 400 mL/day. This is due to the renal tubular injury, which impairs the tubules' ability to reabsorb water and electrolytes, resulting in a concentrated urine. The decreased renal blood flow and subsequent tubular injury also lead to a decrease in glomerular filtration rate (GFR), further contributing to the oliguric state.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperkalemia is a common finding in the oliguric phase of ATN due to the impaired renal excretion of potassium.
**Option B:** Metabolic acidosis is also a characteristic feature of ATN, resulting from the decreased renal excretion of hydrogen ions and the accumulation of lactic acid.
**Option C:** Azotemia, characterized by elevated levels of blood urea nitrogen (BUN) and creatinine, is a hallmark of ATN, reflecting the impaired renal function.

**Clinical Pearl / High-Yield Fact**
In the oliguric phase of ATN, it is essential to promptly address the underlying cause, such as volume depletion or nephrotoxicity, to prevent further renal damage and improve outcomes.

**Correct Answer:** D.