**Core Concept**
In the context of hyponatremia following renal failure, the primary goal is to prevent central pontine myelinolysis (CPM), a potentially life-threatening complication. The osmotic gradient between the serum and intracellular fluid is critical in this scenario, as rapid changes can lead to neuronal damage.

**Why the Correct Answer is Right**
Maintaining serum sodium levels below 130 mEq/L, but not too low, helps to minimize the risk of CPM. When serum sodium levels drop below 120 mEq/L, the risk of CPM increases significantly. In renal failure, the kidneys are unable to regulate sodium and water effectively, leading to fluid overload and hyponatremia. By limiting serum sodium to 120 mEq/L, the risk of CPM can be mitigated while still managing the underlying fluid imbalance.

**Why Each Wrong Option is Incorrect**
**Option B:** Maintaining serum sodium at 125 mEq/L may not be sufficient to prevent CPM, especially in the setting of renal failure where fluid balance is already compromised.

**Option C:** Serum sodium levels of 130 mEq/L may be too high, increasing the risk of CPM, particularly if the underlying cause of hyponatremia is not addressed.

**Option D:** Serum sodium levels of 135 mEq/L are generally considered normal and would not be sufficient to manage hyponatremia in the context of renal failure.

**Clinical Pearl / High-Yield Fact**
In patients with hyponatremia and renal failure, it's essential to carefully monitor serum sodium levels and fluid balance to prevent CPM. A general rule of thumb is to aim for a serum sodium level of 120-125 mEq/L, but this may need to be adjusted based on individual patient factors and underlying disease processes.

**✓ Correct Answer: A. 120 mEq/L**