**Core Concept**
A leukemoid reaction is a massive, transient increase in white blood cells, typically due to severe infection or inflammation, resulting in a left shift in granulocytes—indicating the bone marrow is producing immature neutrophils in response to acute stress. It mimics leukemia but lacks the clonal proliferation seen in true leukemia.

**Why the Correct Answer is Right**
The patient has a severe infection (pulmonary abscess) with *Pseudomonas aeruginosa*, which triggers a robust immune response. The marked leukocytosis (50,000/mL) and a left shift (presence of band forms and metamyelocytes) indicate the bone marrow is hyperactively producing neutrophils to combat infection. This pattern is classic for a leukemoid reaction, not true leukemia, and is a hallmark of severe bacterial infections.

**Why Each Wrong Option is Incorrect**
Option B: Leukopenia refers to low white blood cell counts, which is the opposite of the observed marked leukocytosis.
Option C: Myeloid metaplasia involves abnormal transformation of myeloid cells into non-hematopoietic tissues (e.g., in chronic inflammation), not seen in acute infections.
Option D: Neutrophilia is a rise in neutrophils but does not account for the massive leukocytosis or left shift—leukemoid reaction encompasses both magnitude and immature cell presence.

**Clinical Pearl / High-Yield Fact**
A leukemoid reaction is a key sign of severe bacterial infection (e.g., sepsis, abscess) and should prompt aggressive treatment. It is not a diagnosis of leukemia and resolves with infection control.

✓ Correct Answer: A. Leukemoid reaction