**Core Concept:** Programmed cell death, also known as apoptosis, is a highly regulated and controlled form of cell death essential for maintaining tissue homeostasis, development, and eliminating damaged or unwanted cells. It can be induced by various stimuli including growth factors, cytokines, or cell surface receptors like Fas and TNF receptors.

**Why the Correct Answer is Right:** Apoptosis is a highly regulated process involving multiple steps, including externalization of phosphatidylserine (PS), DNA fragmentation, and membrane blebbing. The correct answer, **Option C: Externalization of phosphatidylserine (PS)**, refers to the key event during apoptosis, where phospholipid phosphatidylserine (PS) is translocated from the inner to the outer leaflet of the cell membrane. This event is recognized by various cells and immune system components, allowing for clearance of the apoptotic cell and preventing inflammation and autoimmunity.

**Why Each Wrong Option is Incorrect:**

A. **Internalization of chromatin (Option A)**: This is an incorrect description of necroptosis, a form of cell death characterized by chromatin condensation and DNA fragmentation, which is different from apoptosis.

B. **Inhibition of DNA replication (Option B)**: Apoptosis does not involve inhibition of DNA replication, as this process is essential for the cell to undergo programmed cell death.

D. **Induction of endonucleases (Option D)**: While endonucleases may be involved in apoptosis, the hallmark described in the question is externalization of phosphatidylserine (PS), not the direct activation of endonucleases.

**Clinical Pearl:** A proper understanding of apoptosis's key events, such as PS externalization, can aid in understanding immune system responses to cell death and the pathogenesis of autoimmune diseases like systemic lupus erythematosus (SLE), where apoptosis is dysregulated, leading to the release of self-antigens and autoimmunity.