**Flow Cytometry Basics**

**Core Concept**
Flow cytometry is a laboratory technique used to analyze and sort cells based on their physical and chemical properties. It involves suspending cells in a fluid and passing them through a laser, which detects and measures various characteristics such as size, granularity, and fluorescence.

**Why the Correct Answer is Right**
Flow cytometry is used to determine the presence and quantity of specific cell surface markers or antigens on cells, which is crucial in diagnosing and monitoring various diseases, including leukemia, lymphoma, and autoimmune disorders. The technique measures the fluorescence emitted by cells that have bound specific fluorescently labeled antibodies to their surface, allowing for the identification and quantification of specific cell populations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because flow cytometry is not primarily used to assess the metabolic activity of cells. While some flow cytometry applications involve measuring cellular metabolism, this is not the primary purpose of the technique.
**Option B:** This option is incorrect because flow cytometry is not typically used to detect the presence of specific proteins within cells. Techniques such as Western blotting or immunohistochemistry are more commonly used for this purpose.
**Option C:** This option is incorrect because flow cytometry is not primarily used to assess the structural integrity of cells. While some flow cytometry applications involve measuring cellular size and granularity, this is not the primary purpose of the technique.

**Clinical Pearl / High-Yield Fact**
Flow cytometry is a highly sensitive and specific technique for diagnosing and monitoring hematological malignancies, and it is widely used in clinical laboratories around the world.

**Correct Answer: D. Determine the presence and quantity of specific cell surface markers or antigens on cells.**