**Core Concept**
Hyponatremia, a condition characterized by an abnormally low sodium level in the blood (Na+ < 135 meq/L), often presents with symptoms like nausea, vomiting, and headaches. In this scenario, the patient's sodium level is 125 meq/L, indicating severe hyponatremia. The primary goal of treatment is to correct the sodium imbalance and prevent further complications.

**Why the Correct Answer is Right**
The initial treatment for severe hyponatremia involves administering hypotonic saline, which contains a lower concentration of sodium than the patient's blood. This helps to slowly correct the sodium imbalance by providing a small amount of sodium while minimizing the risk of rapid increases in osmolality, which can lead to central pontine myelinolysis (CPM). Hypotonic saline is often administered in a controlled manner, typically with a goal of increasing the sodium level by 1-2 meq/L per hour. This approach helps to prevent osmotic shifts and subsequent neurological complications.

**Why Each Wrong Option is Incorrect**
**Option B:** Ultra centrifugation is a laboratory technique used to separate and analyze blood components, but it is not a treatment for hyponatremia.

**Option C:** Haemodialysis may be considered in severe cases of hyponatremia, especially when there is significant water overload, but it is not the initial treatment of choice.

**Option D:** A high protein diet is not a treatment for hyponatremia and may even exacerbate the condition by increasing the renal excretion of sodium.

**Clinical Pearl / High-Yield Fact**
In patients with severe hyponatremia, it is essential to monitor sodium levels closely and adjust treatment accordingly to prevent overcorrection, which can lead to CPM.

**✓ Correct Answer: A. Give hypotonic saline**