**Core Concept**
A leukemoid reaction is a reactive increase in the number of white blood cells (leukocytosis) in response to a severe infection, inflammation, or other stressors, which mimics the appearance of leukemia but is distinct from it. This phenomenon is characterized by a marked increase in the number of immature cells, including band forms, in the peripheral blood.

**Why the Correct Answer is Right**
In this scenario, the patient's body is responding to a severe infection caused by Pseudomonas aeruginosa, which has led to a pronounced increase in the production of white blood cells, particularly neutrophils. This is a classic example of a leukemoid reaction, where the bone marrow is stimulated to produce large numbers of immature cells, including band forms, in an attempt to combat the infection. The marked leukocytosis and presence of numerous immature cells (band forms) in the differential count are characteristic features of this condition. The mechanism underlying a leukemoid reaction involves the activation of the immune system, which leads to the release of cytokines and other inflammatory mediators that stimulate the bone marrow to produce more white blood cells.

**Why Each Wrong Option is Incorrect**
**Option B:** Leukopenia refers to a decrease in the number of white blood cells, which is the opposite of what is observed in this patient.
**Option C:** Myeloid metaplasia refers to the presence of myeloid cells in extramedullary sites, such as the spleen or liver, which is not relevant to this patient's presentation.
**Option D:** Myeloproliferative disease refers to a group of disorders characterized by the excessive production of white blood cells, but it is a distinct clinical entity from a leukemoid reaction.

**Clinical Pearl / High-Yield Fact**
A leukemoid reaction is a diagnosis of exclusion, and it is essential to rule out leukemia and other underlying conditions that may mimic this presentation. A thorough clinical evaluation, including a review of the patient's medical history, physical examination, and laboratory results, is necessary to distinguish a leukemoid reaction from other conditions.

✓ Correct Answer: A. Leukemoid reaction