**Core Concept**
Leukocytosis is a condition characterized by an elevated white blood cell (WBC) count, which can be due to various underlying causes such as infection, inflammation, or leukemia. In this context, a WBC count of more than 100,000 cells/cc of blood is significantly elevated and warrants further investigation.

**Why the Correct Answer is Right**
The correct answer is **A. Leukemia**. Leukemia is a type of cancer that affects the blood and bone marrow, leading to an overproduction of immature WBCs. This can result in a marked increase in the WBC count, often exceeding 100,000 cells/cc of blood. In leukemia, the bone marrow is unable to produce normal WBCs, leading to a buildup of malignant cells.

**Why Each Wrong Option is Incorrect**
* **Option B:** Infections, such as pneumonia or sepsis, can cause leukocytosis, but the WBC count is typically not as high as 100,000 cells/cc of blood.
* **Option C:** Inflammatory conditions, like rheumatoid arthritis, can also lead to leukocytosis, but the WBC count is usually not as elevated as in leukemia.
* **Option D:** This option is not a correct answer, as it is incomplete.

**Clinical Pearl / High-Yield Fact**
A WBC count exceeding 100,000 cells/cc of blood is a red flag for leukemia and warrants an urgent bone marrow biopsy to confirm the diagnosis.

**Correct Answer:** A. Leukemia