**Core Concept**

Flow cytometry is a laboratory technique used to analyze cells or cellular components based on their physical and biochemical properties. It is an important tool in medical research and diagnosis, allowing for the study of cell populations and their characteristics. In the context of this question, flow cytometry is used to measure and characterize cells or cellular components based on their unique properties, such as surface markers, protein expression, and DNA content.

**Why the Correct Answer is Right**

Flow cytometry involves the use of lasers, antibodies, and fluorescent dyes to label cells with specific markers that can identify and differentiate between different cell types. The labeled cells are then passed through a laser beam, which interrogates each cell and measures the fluorescence intensity of the surface markers. The detector records the data, which is then analyzed using specialized software to generate a histogram or scatter plot. Based on these measurements, flow cytometry can determine the size, shape, and internal components of the cells, as well as the expression of specific proteins or antigens on their surface.

**Why Each Wrong Option is Incorrect**

A. This option is incorrect because flow cytometry does not focus on measuring cell size and shape alone. It provides more comprehensive information by assessing cellular components and their markers.

B. Although flow cytometry can detect DNA content, it is not the primary purpose of this technique. It primarily focuses on assessing cell surface and intracellular markers.

C. This option is incorrect because flow cytometry does not primarily measure cell viability or function. Its main focus is on the identification and characterization of cells based on their surface and intracellular markers.

D. While flow cytometry can detect specific cellular components and surface markers, it is not limited to measuring only the number of cells. The technique provides a more comprehensive assessment of cells, including their size, shape, and marker expression.

**Clinical Pearl / High-Yield Fact**

Flow cytometry has numerous applications in various medical fields, including:

1. Diagnosis and monitoring of infections, cancers, and immune system disorders.
2. Research on cell differentiation, activation, and function.
3. Drug development and efficacy testing.
4. Prenatal diagnosis of genetic disorders and fetal aneuploidy.
5. Monitoring of stem cell transplantation and immune system reconstitution.

**Correct Answer Explanation**

The correct answer is option D: Flow cytometry measures cells, their size, and marker expression. It is essential to understand that flow cytometry is a versatile technique that provides comprehensive information about cells, beyond just counting them. By assessing cell size, shape, and marker expression, flow cytometry helps in diagnosing diseases, monitoring immune system function, and studying cellular processes in research settings.