**Core Concept**
Irreversible cell injury is a type of cell damage that cannot be reversed, often resulting from severe cellular stress, such as ischemia, toxins, or infections. This process involves a cascade of cellular changes, including mitochondrial dysfunction, cellular swelling, and eventual cell death. The hallmark of irreversible cell injury is the loss of cellular integrity and function.

**Why the Correct Answer is Right**
Irreversible cell injury is characterized by the loss of membrane potential, leading to an influx of calcium ions and activation of various cellular enzymes. This ultimately results in the breakdown of cellular structures and the release of intracellular contents into the surrounding tissue space. The correct answer reflects this loss of cellular integrity and function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because reversible cell injury is typically characterized by cellular swelling and a return to normal function upon removal of the underlying stress. Irreversible cell injury, on the other hand, involves permanent cellular damage.

**Option B:** This option is incorrect because apoptosis, or programmed cell death, is a form of cell death that is often reversible through various cellular mechanisms. Irreversible cell injury, however, is a more severe and irreversible process.

**Clinical Pearl / High-Yield Fact**
One key distinction between reversible and irreversible cell injury is the presence of cellular swelling. Reversible cell injury typically involves mild cellular swelling, whereas irreversible cell injury is characterized by severe cellular swelling and loss of cellular integrity.

**Correct Answer:** D. Cellular swelling and loss of membrane integrity are hallmark features of irreversible cell injury.