**Core Concept**
Phosphatidylserine is a phospholipid component of the cell membrane. During apoptosis, a process of programmed cell death, the normal asymmetric distribution of phospholipids across the cell membrane is disrupted, leading to the exposure of phosphatidylserine on the outer surface of the cell. This exposure serves as a signal for phagocytic cells to recognize and engulf the apoptotic cell.

**Why the Correct Answer is Right**
Phosphatidylserine is specifically translocated to the outer leaflet of the plasma membrane during apoptosis, where it can be recognized by phagocytic receptors, such as CD36 and annexin V. This process is mediated by a family of proteins called scramblases, which are activated during apoptosis and facilitate the movement of phosphatidylserine across the membrane. In contrast, other phospholipids, such as phosphatidylinositol and phosphatidylethanolamine, are not exposed on the cell surface during apoptosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Phosphatidylinositol is a phospholipid involved in signaling pathways, including the phosphoinositide 3-kinase (PI3K) pathway. However, it is not a marker of apoptosis.
**Option C:** Phosphatidylcholine is a major component of cell membranes, but it is not specifically exposed on the outer surface of cells undergoing apoptosis.
**Option D:** Phosphatidylethanolamine is also a component of cell membranes, but it is not a marker of apoptosis.

**Clinical Pearl / High-Yield Fact**
The exposure of phosphatidylserine on the outer surface of apoptotic cells serves as a signal for phagocytic cells to recognize and engulf the cell, preventing the release of inflammatory mediators and avoiding tissue damage. This process is critical for maintaining tissue homeostasis and preventing chronic inflammation.

**✓ Correct Answer: B. Phosphatidyl serine**