**Core Concept**
Hypernatremia is a condition characterized by an elevated serum sodium concentration, typically above 145 mmol/L. This imbalance can lead to cellular dehydration, causing various neurological and physical symptoms. The underlying pathophysiology involves the loss of water relative to sodium, leading to increased serum osmolality and subsequent cellular shrinkage.

**Why the Correct Answer is Right**
Hypernatremia can cause severe symptoms, including:
* **Dehydration of brain cells**, leading to confusion, altered mental status, and even seizures.
* **Thirst and polydipsia**, as the body attempts to compensate for the sodium imbalance.
* **Muscle weakness**, due to the effects of sodium on muscle function.
* **Oliguria**, as the kidneys attempt to conserve water and concentrate urine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may be a feature of hypernatremia, as dehydration of brain cells can lead to impaired cognitive function.
**Option B:** This option is a common cause of hypernatremia, as it can lead to water loss and sodium retention.
**Option C:** While this option may be associated with hypernatremia, it is not a direct feature of the condition.

**Clinical Pearl / High-Yield Fact**
A key aspect of managing hypernatremia is to correct the sodium imbalance gradually, as rapid changes can lead to cerebral edema and worsen symptoms.

**Correct Answer: D.**