**Core Concept:**
Natural Killer (N.K.) cells and cytotoxic cells are types of immune cells that play a crucial role in the body's defense against virus-infected cells and cancerous cells. They are part of the innate immune system and help eliminate pathogens and abnormal cells. Cytotoxic cells, also known as T cells (T-lymphocytes), are a subset of lymphocytes that are involved in cell-mediated immunity.

**Why the Correct Answer is Right:**
The correct answer, **C**. Natural Killer (N.K.) cells and cytotoxic cells are differentiated by their cytotoxic mechanism. N.K. cells primarily target and eliminate virus-infected cells, while cytotoxic cells primarily target and eliminate cancerous cells. Both types of cells exert their cytotoxic effects through the release of cytotoxic granules containing perforin and granzymes, which create pores in the target cell membrane and induce apoptosis, a form of programmed cell death.

**Why Each Wrong Option is Incorrect:**
A. Perforin and granzymes are cytotoxic molecules released by N.K. cells and cytotoxic cells, not distinguishing factors between them.
B. The presence of CD8+ markers is common to both N.K. cells and cytotoxic cells, making this a poor basis for differentiation.
D. While cytotoxic cells (T cells) do play a role in combating viral infections, this option does not differentiate between N.K. cells and cytotoxic cells.

**Why the Correct Answer is Right (again):**
Cytotoxic cells (cytotoxic T-lymphocytes) are a subset of lymphocytes with a primary role in defending against virus-infected cells, while N.K. cells (natural killer cells) primarily target and eliminate cancerous cells. Both types of cells utilize perforin and granzymes to induce apoptosis in target cells, but their primary functions differ.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because perforin and granzymes are cytotoxic molecules released by both N.K. cells and cytotoxic cells, not distinguishing factors between them.
B. The presence of CD8+ markers is common to both N.K. cells and cytotoxic cells, making this a poor basis for differentiation.
D. While cytotoxic cells (T cells) do play a role in combating viral infections, this option does not differentiate between N.K. cells and cytotoxic cells.

**Clinical Pearls:**
1. N.K. cells and cytotoxic cells are part of the immune system's defense against viral infections and cancer.
2. Understanding the differences in their functions helps guide treatment strategies for patients with virus infections and cancer.
3. Apoptosis is a critical mechanism for eliminating virus-infected and cancerous cells, highlighting the significance of these immune cells in clinical settings.