**Core Concept**
The process of irreversible cell injury involves a series of biochemical changes that ultimately lead to cell death. This process is often initiated by injury to the cell membrane, which can disrupt the cell's ability to regulate its internal environment.

**Why the Correct Answer is Right**
In irreversible cell injury, the influx of **Calcium** ions (Ca2+) plays a critical role. Once inside the cell, calcium ions activate various enzymes and cellular processes that lead to the breakdown of cellular components and ultimately, cell death. This is because calcium ions interact with and activate several key enzymes, including phospholipase A2, which breaks down phospholipids in the cell membrane, and proteases, which degrade cellular proteins.

**Why Each Wrong Option is Incorrect**
**Option A:** **Sodium** ions (Na+) are important in maintaining the cell's osmotic balance and in the transmission of nerve impulses, but they are not directly involved in the process of irreversible cell injury.
**Option B:** **Potassium** ions (K+) are also important in maintaining the cell's osmotic balance and in the transmission of nerve impulses, but they are not directly involved in the process of irreversible cell injury.
**Option C:** **Magnesium** ions (Mg2+) are important in many cellular processes, including the regulation of enzyme activity and the maintenance of cellular homeostasis, but they are not directly involved in the process of irreversible cell injury.

**Clinical Pearl / High-Yield Fact**
Remember that the influx of calcium ions is a key event in the process of irreversible cell injury, and that this process can be triggered by a variety of insults, including ischemia, toxins, and physical injury.

**Correct Answer:** C. Calcium.