**Core Concept**
The question is testing knowledge on the complications associated with dialysis, a life-sustaining treatment for patients with end-stage renal disease (ESRD). The most common complication of dialysis is related to the dialysis process itself, which can lead to various systemic effects.

**Why the Correct Answer is Right**
The correct answer is **A. Hyperkalemia**, which is a potentially life-threatening complication of dialysis. Hyperkalemia occurs due to the imbalance of potassium levels in the body, which can lead to cardiac arrhythmias and even cardiac arrest. During dialysis, potassium is removed from the blood, but if the dialysis process is not properly managed, potassium levels can drop too low, leading to hyperkalemia. This is often referred to as "hypotonicity" of the dialysate, which can lead to the rapid removal of potassium from the body.

**Why Each Wrong Option is Incorrect**
* **Option B:** **Infection** is a common complication of dialysis, but it is not the most common complication. Infections can occur due to the insertion of catheters or needles for dialysis, but they are not as frequent as hyperkalemia.
* **Option C:** **Hypotension** is a common side effect of dialysis, but it is not the most common complication. Hypotension can occur due to the rapid removal of fluid from the body, but it is not as severe as hyperkalemia.
* **Option D:** **Fluid overload** is a complication of dialysis, but it is not the most common complication. Fluid overload can occur if the dialysis process is not properly managed, leading to an accumulation of fluid in the body.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that hyperkalemia is a common complication of dialysis, and it can be prevented by proper management of the dialysis process, including the use of a potassium-sparing dialysate and careful monitoring of potassium levels.

**Correct Answer:** A. Hyperkalemia