**Core Concept**
The underlying principle being tested is the relationship between electrolyte imbalances and their effects on gastrointestinal motility, particularly in the context of paralytic ileus. **Hypokalemia** is a well-known cause of paralytic ileus due to its impact on smooth muscle function. Electrolyte imbalances can significantly affect the functioning of the gastrointestinal tract.

**Why the Correct Answer is Right**
Hypokalemia, or low potassium levels, disrupts the normal functioning of smooth muscle cells and nerve conduction, leading to a decrease in intestinal motility. This is because potassium is crucial for maintaining the resting membrane potential of cells, including those in the gastrointestinal tract. When potassium levels are low, it can lead to a failure of the intestinal muscles to contract, resulting in paralytic ileus.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperkalemia can lead to cardiac arrhythmias and muscle weakness but is less commonly associated with paralytic ileus compared to hypokalemia.
**Option B:** Hyponatremia can cause neurological symptoms and fluid balance issues but is not the most common electrolyte imbalance causing paralytic ileus.
**Option D:** Hypernatremia can lead to dehydration and neurological issues but is not typically associated with paralytic ileus.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that hypokalemia can be caused by various factors, including excessive potassium loss through the gastrointestinal tract, kidneys, or skin, and that correcting the underlying cause is essential in managing paralytic ileus.

**Correct Answer:** D. Hypokalemia.