**Core Concept**
Leucopenia refers to a reduction in the total leucocyte count, which is a critical parameter in assessing the immune status of a patient. The total leucocyte count includes various types of white blood cells, such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

**Why the Correct Answer is Right**
The term "leucopenia" is derived from the Greek words "leukos," meaning white, and "penia," meaning deficiency. A decrease in the total leucocyte count can be caused by various factors, including bone marrow failure, decreased production of white blood cells, increased peripheral destruction, or sequestration. For example, chemotherapy and radiation therapy can lead to leucopenia by damaging the bone marrow and impairing its ability to produce white blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe a reduction in the total leucocyte count.
**Option B:** This option is incorrect because it describes a condition where there is an increase in the total leucocyte count, not a decrease.
**Option C:** This option is incorrect because it refers to a specific type of white blood cell, but does not accurately describe a reduction in the total leucocyte count.

**Clinical Pearl / High-Yield Fact**
It's essential to note that leucopenia can increase the risk of infections, as the body's immune system is compromised. Therefore, patients with leucopenia require careful monitoring and management to prevent infections.

**Correct Answer: C. Leucopenia.**