**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.**