**Core Concept**
The oliguric phase of Acute Renal Failure (ARF) is characterized by a sudden decrease in urine output, typically less than 400 mL/day. This phase is marked by fluid overload, electrolyte imbalances, and acid-base disturbances.

**Why the Correct Answer is Right**
During the oliguric phase of ARF, hypokalemia is not a characteristic feature. In fact, hyperkalemia is more commonly observed due to the accumulation of potassium in the body as a result of decreased renal excretion. The kidneys play a crucial role in maintaining potassium homeostasis, and their dysfunction leads to hyperkalemia. Hyperkalemia can cause cardiac arrhythmias, muscle weakness, and even cardiac arrest.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest pain is not a direct characteristic of the oliguric phase of ARF, although it may be associated with underlying conditions such as pulmonary edema or cardiac disease.
**Option B:** Acidosis is a common feature of the oliguric phase of ARF due to the accumulation of metabolic byproducts and the kidneys' inability to excrete hydrogen ions.
**Option C:** Hypertension is a possible complication of ARF, especially in the context of fluid overload and activation of the renin-angiotensin-aldosterone system.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the oliguric phase of ARF is a critical period that requires prompt recognition and management to prevent further complications. Monitoring of electrolytes, particularly potassium levels, is crucial to avoid cardiac arrhythmias and other life-threatening complications.

**✓ Correct Answer: D. Hypokalemia**