## Core Concept
Natural Killer (NK) cells are a type of lymphocyte that play a crucial role in the innate immune system, particularly in the defense against viral infections and in the surveillance against tumor cells. They are known for their ability to recognize and kill infected cells or tumor cells without prior sensitization. NK cells are characterized by the presence of CD56 and the absence of CD3.

## Why the Correct Answer is Right
The correct answer, which is not provided, would relate to a characteristic or function that is not true for NK cells. Generally, NK cells are known for:
- Their role in cell-mediated cytotoxicity.
- Production of cytokines like IFN-gamma.
- Expression of specific surface markers.

## Why Each Wrong Option is Incorrect
Given that the specific details of the options (A, B, C, D) are not provided, let's hypothetically analyze:
- **Option A:** If it states a true characteristic of NK cells, such as their ability to induce cell death in target cells through the release of cytotoxins (e.g., perforin and granzymes), then it would be incorrect as an "except" option.
- **Option B:** If it mentions a function or feature accurately attributed to NK cells, like their role in antibody-dependent cellular cytotoxicity (ADCC), it would not be the correct "except" answer.
- **Option C:** Similarly, if it correctly describes NK cell involvement in immune responses, such as the early defense against viral infections, it would be incorrect as the "except" option.
- **Option D:** This would be the correct answer because it presumably states something not true about NK cells.

## Clinical Pearl / High-Yield Fact
A key point to remember about NK cells is their **deficiency** association with increased susceptibility to viral infections and possibly certain types of cancer. A notable condition related to NK cell deficiency is the **Hemophagocytic Lymphohistiocytosis (HLH)**, where NK cell function is impaired, leading to uncontrolled immune responses.

## Correct Answer Line
**Correct Answer: D.**