A 50-year-old woman is discovered to have metastatic breast cancer. One week after receiving her first dose of chemotherapy, she develops bacterial pneumonia. Which of the following best explains this patient’s susceptibility to bacterial infection?

**Core Concept**
The patient's chemotherapy-induced neutropenia increases her susceptibility to bacterial infections due to a significant decrease in neutrophil count, compromising the body's defense mechanisms against pathogens.

**Why the Correct Answer is Right**
Chemotherapy causes bone marrow suppression, leading to a reduction in the production of neutrophils, a type of white blood cell crucial for fighting infections. As a result, the patient's immune system is weakened, making her more prone to bacterial infections, including pneumonia. Neutrophils play a vital role in phagocytosing and eliminating pathogens, and their deficiency compromises the body's ability to respond to infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Chemotherapy-induced thrombocytopenia would primarily increase the risk of bleeding rather than bacterial infections.
**Option B:** Although chemotherapy can affect the immune system, the primary mechanism is not through the disruption of the thymus.
**Option C:** The patient's susceptibility to bacterial infection is not directly related to her age.

**Clinical Pearl / High-Yield Fact**
Remember that chemotherapy-induced neutropenia is a common cause of infections in cancer patients, and prompt administration of antibiotics is crucial in such cases.

**Correct Answer: C. The patient's susceptibility to bacterial infection is due to chemotherapy-induced neutropenia.**

✓ Correct Answer: D. Neutropenia