**Core Concept**
Acute hyponatremia is a condition characterized by a rapid decline in serum sodium levels, leading to an imbalance in the body's osmolality. This can cause water to shift into cells, leading to swelling of brain cells and potentially life-threatening complications.

**Why the Correct Answer is Right**
The development of symptoms in acute hyponatremia is related to the rate of sodium decline. A rapid decline in serum sodium can lead to cerebral edema and subsequent symptoms such as headache, nausea, vomiting, and seizures. The exact rate of sodium decline that leads to symptoms is not well-defined, but a commonly cited threshold is a drop of 8-10 mmol/L in 24 hours. This rapid change in osmolality disrupts the normal balance of electrolytes and water in the brain, leading to swelling of brain cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the development of symptoms in acute hyponatremia is not solely dependent on the absolute level of serum sodium. Rather, it is the rate of decline that is more predictive of symptom development.
**Option B:** This option is incorrect because the development of symptoms in acute hyponatremia is not solely dependent on the duration of hyponatremia. Symptoms can occur even in the absence of prolonged hyponatremia if the rate of sodium decline is rapid enough.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the development of symptoms in acute hyponatremia is often related to the rate of sodium decline, rather than the absolute level of serum sodium. This is why a rapid drop in sodium levels, even from a relatively high starting point, can be more dangerous than a gradual decline from a lower starting point.

**Correct Answer: C. 8-10 mmol/L in 24 hours.**