**Core Concept**
Mild dehydration is characterized by a decrease in body water content, typically less than 5% of the body weight. This condition can lead to various physiological changes, including alterations in blood volume, electrolyte imbalances, and changes in urine concentration.

**Why the Correct Answer is Right**
The presence of mild dehydration is often associated with several clinical features, including:
* Decreased urine output, which can range from normal to slightly reduced.
* Concentrated urine, which can be detected on urinalysis.
* Increased thirst, which is a common symptom in mild dehydration.
* Tachycardia, or an increased heart rate, due to decreased blood volume.
* Mild orthostatic hypotension, or a decrease in blood pressure when standing.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as mild dehydration does not typically cause a significant increase in blood urea nitrogen (BUN). BUN levels may actually decrease due to decreased blood volume.
**Option B:** This option is incorrect as mild dehydration is not typically associated with a significant decrease in serum osmolality. Serum osmolality may actually increase due to water loss.
**Option C:** This option is incorrect as mild dehydration can cause a decrease in serum sodium levels, not an increase. This is due to the body's attempt to retain water and dilute the sodium concentration.

**Clinical Pearl / High-Yield Fact**
It's essential to note that mild dehydration can be challenging to diagnose, as the symptoms may be nonspecific and overlap with other conditions. A careful history and physical examination, combined with laboratory tests, can help diagnose and manage mild dehydration.

**Correct Answer: C. An increase in serum sodium levels.**