**Core Concept**
Flow cytometry is a laboratory technique used to analyze and sort cells based on their physical and chemical characteristics. It is particularly useful in the diagnosis and monitoring of hematological disorders. Leukocytosis, an increase in the number of white blood cells in the blood, can be detected through flow cytometry by analyzing the morphology, size, and fluorescence of white blood cells.

**Why the Correct Answer is Right**
Leukocytosis can be caused by various conditions, including infections, inflammation, and leukemia. Flow cytometry can help identify the type of white blood cells that are increased, such as neutrophils, lymphocytes, or monocytes. This information is crucial in diagnosing and managing hematological disorders. The technique involves labeling the cells with fluorescent dyes that bind to specific cell surface antigens, allowing for the identification of abnormal cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Polycythemia refers to an increase in the number of red blood cells, which can be detected through complete blood counts (CBCs) and other laboratory tests, but not specifically through flow cytometry.

**Option B:** Thrombocytosis, an increase in platelet count, can also be detected through CBCs, but it is not typically analyzed through flow cytometry.

**Option D:** Neutrophilia, an increase in neutrophil count, is a specific type of leukocytosis, but flow cytometry is a more comprehensive technique that can detect changes in multiple types of white blood cells.

**Clinical Pearl / High-Yield Fact**
Flow cytometry is a valuable tool in the diagnosis and management of hematological disorders, particularly in the identification of abnormal cells and the detection of minimal residual disease in leukemia patients.

**✓ Correct Answer: C. Leukocytosis**