**Core Concept**
Irreversible cell injury is a type of cellular damage that occurs when cells are subjected to severe stress, leading to the breakdown of cellular structures and ultimately, cell death. This process is characterized by the activation of various cellular pathways, including the caspase cascade, which results in the fragmentation of DNA and the disintegration of cellular organelles.

**Why the Correct Answer is Right**
Irreversible cell injury is typically indicated by the loss of cell membrane integrity, which allows ions and water to flood into the cell, causing it to swell and ultimately, rupture. This process is often accompanied by the activation of lysosomes, which release digestive enzymes that break down cellular components. The hallmark of irreversible cell injury is the loss of cellular ATP production, which is essential for maintaining cellular homeostasis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may refer to reversible cell injury, which is characterized by the activation of cellular stress pathways and the expression of heat shock proteins.
**Option B:** This option may refer to apoptosis, a type of programmed cell death that is typically characterized by the activation of caspases and the fragmentation of DNA.
**Option C:** This option may refer to necrosis, a type of cell death that is typically characterized by the loss of cellular membrane integrity, but is not necessarily irreversible.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the distinction between reversible and irreversible cell injury is critical in clinical practice, as it can impact treatment decisions and patient outcomes.

**Correct Answer: D. Loss of cellular membrane integrity.**